[xabt] fix NRT annotations in MSBuild Tasks (#10283)

Context: #10277

This PR systematically modernizes MSBuild task classes in
`src/Xamarin.Android.Build.Tasks/Tasks/` to follow the repository's
nullable reference types guidelines as outlined in
`copilot-instructions.md`.

## Changes Made

### Phase 1: Files with `#nullable disable` (10/22 completed - 45%)
Converted files from `#nullable disable` to `#nullable enable` and
modernized properties:

**Completed files:**
- AndroidApkSigner.cs, CalculateProjectDependencies.cs,
  JavaToolTask.cs (base class)
- ConvertCustomView.cs, MergeRemapXml.cs, ImportJavaDoc.cs  
- JavaDoc.cs, JavaSourceUtils.cs, StripNativeLibraries.cs,
  LinkApplicationSharedLibraries.cs

### Phase 2: Files without nullable directives (27/117 completed - 23%)
Added `#nullable enable` directive and converted patterns:

**Completed files:**
- Aapt2Compile.cs, AndroidMessage.cs, AndroidAdb.cs,
  AndroidSignPackage.cs
- Aapt2LinkAssetPack.cs, AdjustJavacVersionArguments.cs,
  AndroidCreateDebugKey.cs
- AndroidError.cs, AndroidWarning.cs, AndroidComputeResPaths.cs,
  AndroidDotnetToolTask.cs
- AndroidZipAlign.cs, AppendCustomMetadataToItemGroup.cs,
  AssemblyModifierPipeline.cs
- CheckClientHandlerType.cs, CheckDuplicateJavaLibraries.cs,
  CheckForInvalidDesignerConfig.cs
- CheckProjectItems.cs, ClassParse.cs,
  CollectAssemblyFilesForArchive.cs, ComputeHash.cs
- CopyIfChanged.cs, CopyResource.cs, CreateAssemblyStore.cs,
  CollectDalvikFilesForArchive.cs
- D8.cs, CreateTemporaryDirectory.cs

## Progress Summary

- **Total files modernized:** 37 out of 139 target files (27%
  complete)
- **Phase 1 progress:** 10/22 files with `#nullable disable` (45%
  complete)
- **Phase 2 progress:** 27/117 files without nullable directives (23%
  complete)
- **Patterns established:** Clear, systematic approach for remaining
  files

## Impact

- Improves type safety across MSBuild task properties
- Follows established repository conventions for nullable reference
  types
- Maintains backward compatibility while modernizing the codebase
- Establishes foundation for completing remaining files

#10277 is not complete, but we will try another pass in a future PR.

Co-authored-by: Jonathan Peppers <[email protected]>

Author: Copilot

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Compile.cs

@@ -1,4 +1,6 @@
 // Copyright (C) 2011 Xamarin, Inc. All rights reserved.
+#nullable enable
+
 using System;
 using System.Diagnostics;
 using System.IO;

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2LinkAssetPack.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Diagnostics;
 using System.IO;

src/Xamarin.Android.Build.Tasks/Tasks/AdjustJavacVersionArguments.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using Microsoft.Build.Utilities;
 using Microsoft.Build.Framework;

src/Xamarin.Android.Build.Tasks/Tasks/AndroidAdb.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using Microsoft.Android.Build.Tasks;
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
@@ -34,7 +36,7 @@ protected override string GenerateFullPathToTool ()
 		protected override string GenerateCommandLineCommands ()
 		{
 			var sb = new StringBuilder ();
-			if (!string.IsNullOrEmpty (AdbTarget))
+			if (!AdbTarget.IsNullOrEmpty ())
 				sb.Append ($" {AdbTarget} ");
 			sb.AppendFormat ("{0} {1}", Command, Arguments);
 			return sb.ToString ();

src/Xamarin.Android.Build.Tasks/Tasks/AndroidApkSigner.cs

@@ -1,4 +1,4 @@
-#nullable disable
+#nullable enable
 
 using System;
 using System.IO;
@@ -14,21 +14,21 @@ public class AndroidApkSigner : JavaToolTask
 		public override string TaskPrefix => "AAS";
 
 		[Required]
-		public string ApkSignerJar { get; set; }
+		public string ApkSignerJar { get; set; } = "";
 
 		[Required]
-		public string ApkToSign { get; set; }
+		public string ApkToSign { get; set; } = "";
 
 		[Required]
-		public ITaskItem ManifestFile { get; set; }
+		public ITaskItem ManifestFile { get; set; } = null!;
 
-		public string KeyStore { get; set; }
+		public string? KeyStore { get; set; }
 
-		public string KeyAlias { get; set; }
+		public string? KeyAlias { get; set; }
 
-		public string PlatformKey { get; set; }
+		public string? PlatformKey { get; set; }
 
-		public string PlatformCert { get; set; }
+		public string? PlatformCert { get; set; }
 
 		/// <summary>
 		/// The Password for the Key.
@@ -39,7 +39,7 @@ public class AndroidApkSigner : JavaToolTask
 		///   env:<PasswordEnvironentVariable>
 		///   file:<PasswordFile>
 		/// </summary>
-		public string KeyPass { get; set; } = string.Empty;
+		public string KeyPass { get; set; } = "";
 
 		/// <summary>
 		/// The Password for the Keystore.
@@ -50,15 +50,15 @@ public class AndroidApkSigner : JavaToolTask
 		///   env:<PasswordEnvironentVariable>
 		///   file:<PasswordFile>
 		/// </summary>
-		public string StorePass { get; set; } = string.Empty;
+		public string StorePass { get; set; } = "";
 
-		public string AdditionalArguments { get; set; }
+		public string? AdditionalArguments { get; set; }
 
 		void AddStorePass (CommandLineBuilder cmd, string cmdLineSwitch, string value)
 		{
-			string pass = value.Replace ("env:", string.Empty)
-				.Replace ("file:", string.Empty)
-				.Replace ("pass:", string.Empty);
+			string pass = value.Replace ("env:", "")
+				.Replace ("file:", "")
+				.Replace ("pass:", "");
 			if (value.StartsWith ("env:", StringComparison.Ordinal)) {
 				cmd.AppendSwitchIfNotNull ($"{cmdLineSwitch} env:", pass);
 			}
@@ -74,20 +74,21 @@ protected override string GenerateCommandLineCommands ()
 			var cmd = new CommandLineBuilder ();
 
 			var manifest = AndroidAppManifest.Load (ManifestFile.ItemSpec, MonoAndroidHelper.SupportedVersions);
-			int minSdk = MonoAndroidHelper.SupportedVersions.MinStableVersion.ApiLevel;
-			int maxSdk = MonoAndroidHelper.SupportedVersions.MaxStableVersion.ApiLevel;
+			int? minSdk = MonoAndroidHelper.SupportedVersions.MinStableVersion?.ApiLevel;
+			int? maxSdk = MonoAndroidHelper.SupportedVersions.MaxStableVersion?.ApiLevel;
 			if (manifest.MinSdkVersion.HasValue)
 				minSdk = manifest.MinSdkVersion.Value;
 
 			if (manifest.TargetSdkVersion.HasValue)
 				maxSdk = manifest.TargetSdkVersion.Value;
 
-			minSdk = Math.Min (minSdk, maxSdk);
+			if (minSdk.HasValue && maxSdk.HasValue)
+				minSdk = Math.Min (minSdk.Value, maxSdk.Value);
 
 			cmd.AppendSwitchIfNotNull ("-jar ", ApkSignerJar);
 			cmd.AppendSwitch ("sign");
 
-			if (!string.IsNullOrEmpty (PlatformKey) && !string.IsNullOrEmpty (PlatformCert)) {
+			if (!PlatformKey.IsNullOrEmpty () && !PlatformCert.IsNullOrEmpty ()) {
 				cmd.AppendSwitchIfNotNull ("--key ", PlatformKey);
 				cmd.AppendSwitchIfNotNull ("--cert ", PlatformCert);
 			} else {
@@ -97,11 +98,15 @@ protected override string GenerateCommandLineCommands ()
 				AddStorePass (cmd, "--key-pass", KeyPass);
 			}
 
-			cmd.AppendSwitchIfNotNull ("--min-sdk-version ", minSdk.ToString ());
-			cmd.AppendSwitchIfNotNull ("--max-sdk-version ", maxSdk.ToString ());
+			if (minSdk is not null) {
+				cmd.AppendSwitchIfNotNull ("--min-sdk-version ", minSdk.ToString ());
+			}
+			if (maxSdk is not null) {
+				cmd.AppendSwitchIfNotNull ("--max-sdk-version ", maxSdk.ToString ());
+			}
 
 
-			if (!string.IsNullOrEmpty (AdditionalArguments))
+			if (!AdditionalArguments.IsNullOrEmpty ())
 				cmd.AppendSwitch (AdditionalArguments);
 
 			cmd.AppendSwitchIfNotNull (" ", ApkToSign);
@@ -129,7 +134,7 @@ protected override string ToolName {
 
 		protected override bool ValidateParameters ()
 		{
-			if (!string.IsNullOrEmpty (PlatformKey) && !string.IsNullOrEmpty (PlatformCert)) {
+			if (!PlatformKey.IsNullOrEmpty () && !PlatformCert.IsNullOrEmpty ()) {
 				if (!File.Exists (PlatformKey)) {
 					Log.LogCodedError ("XA4310", Properties.Resources.XA4310, "$(AndroidSigningPlatformKey)", PlatformKey);
 					return false;
@@ -139,15 +144,15 @@ protected override bool ValidateParameters ()
 					return false;
 				}
 			} else {
-				if (!string.IsNullOrEmpty (KeyStore) && !File.Exists (KeyStore)) {
+				if (!KeyStore.IsNullOrEmpty () && !File.Exists (KeyStore)) {
 					Log.LogCodedError ("XA4310", Properties.Resources.XA4310, "$(AndroidSigningKeyStore)", KeyStore);
 					return false;
 				}
-				if (string.IsNullOrEmpty (KeyPass)) {
+				if (KeyPass.IsNullOrEmpty ()) {
 					Log.LogCodedError ("XA4314", Properties.Resources.XA4314, "$(AndroidSigningKeyPass)");
 					return false;
 				}
-				if (string.IsNullOrEmpty (StorePass)) {
+				if (StorePass.IsNullOrEmpty ()) {
 					Log.LogCodedError ("XA4314", Properties.Resources.XA4314, "$(AndroidSigningStorePass)");
 					return false;
 				}

src/Xamarin.Android.Build.Tasks/Tasks/AndroidComputeResPaths.cs

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Text;
 using Microsoft.Build.Utilities;

src/Xamarin.Android.Build.Tasks/Tasks/AndroidCreateDebugKey.cs

@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.IO;
 using System.Resources;
 using Microsoft.Build.Framework;

src/Xamarin.Android.Build.Tasks/Tasks/AndroidDotnetToolTask.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.IO;
 using System.Runtime.InteropServices;
@@ -106,13 +108,13 @@ string FindDotnet ()
 		string FindMono ()
 		{
 			string mono = BuildEngine4.GetRegisteredTaskObjectAssemblyLocal<string> (MonoKey, Lifetime);
-			if (!string.IsNullOrEmpty (mono)) {
+			if (!mono.IsNullOrEmpty ()) {
 				Log.LogDebugMessage ($"Found cached mono via {nameof (BuildEngine4.RegisterTaskObject)}");
 				return mono;
 			}
 
 			var env = Environment.GetEnvironmentVariable ("PATH");
-			if (string.IsNullOrEmpty (env)) {
+			if (!env.IsNullOrEmpty ()) {
 				foreach (var path in env.Split (Path.PathSeparator)) {
 					if (File.Exists (mono = Path.Combine (path, "mono"))) {
 						Log.LogDebugMessage ("Found mono in $PATH");
@@ -138,7 +140,7 @@ string FindMono ()
 		protected virtual CommandLineBuilder GetCommandLineBuilder ()
 		{
 			var cmd = new CommandLineBuilder ();
-			if (!string.IsNullOrEmpty (AssemblyPath)) {
+			if (!AssemblyPath.IsNullOrEmpty ()) {
 				cmd.AppendFileNameIfNotNull (AssemblyPath);
 			}
 			return cmd;

src/Xamarin.Android.Build.Tasks/Tasks/AndroidError.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
 using Microsoft.Android.Build.Tasks;

src/Xamarin.Android.Build.Tasks/Tasks/AndroidMessage.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
 using Microsoft.Android.Build.Tasks;