SLVS-2964 fix: resolve 5 SonarQube code quality issues

src/Integration.Vsix/ErrorList/IssuesSnapshot.cs

@@ -259,7 +259,7 @@ private string GetErrorCategory(IAnalysisIssue issue)
         /// Returns true/false if the ErrorList should hide the requested issue.
         /// </summary>
         /// <returns>Returns true if the issue doesn't have a valid span.</returns>
-        private bool ShouldHideIssue(IAnalysisIssueVisualization issue)
+        private static bool ShouldHideIssue(IAnalysisIssueVisualization issue)
         {
             if (issue.IsFileLevel())
             {

src/Integration.Vsix/SonarLintTagger/CancellableJobRunner.cs

@@ -160,7 +160,7 @@ private async System.Threading.Tasks.Task ExecuteAsync()
                 // We're cancelling here purely for testing purposes: tests can wait on the token WaitHandle
                 // to know that the job runner has finished. Ugly, but it means we can write reliable tests.
                 cancellationSource.Cancel();
-                cancellationSource?.Dispose();
+                cancellationSource.Dispose();
                 cancellationSource = null;
             }
         }
@@ -174,7 +174,7 @@ public void Cancel()
                 {
                     State = JobRunnerProgress.RunnerState.Cancelled;
                     logger.WriteLine(Strings.JobRunner_CancellingJob, jobDescription, startTime.ToLongTimeString());
-                    cancellationSource?.Cancel();
+                    cancellationSource.Cancel();
                     progress?.Report(new JobRunnerProgress(State, completedOperations, totalOperations));
                 }
             }

src/Integration/WPF/NotifyErrorViewModelBase.cs

@@ -20,6 +20,7 @@
 
 using System.Collections;
 using System.ComponentModel;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using SonarLint.VisualStudio.Core.WPF;
 
@@ -40,7 +41,7 @@ IEnumerable INotifyDataErrorInfo.GetErrors(string propertyName)
             {
                 return new[] { error };
             }
-            return null;
+            return Enumerable.Empty<string>();
         }
 
         #endregion

src/IssueViz/Models/AnalysisIssueVisualizationConverter.cs

@@ -80,7 +80,7 @@ private void CalculateSpanForSecondaryLocationsInSameFile(IAnalysisIssueVisualiz
             }
         }
 
-        private IReadOnlyList<IAnalysisIssueFlowVisualization> Convert(IEnumerable<IAnalysisIssueFlow> flows)
+        private static IReadOnlyList<IAnalysisIssueFlowVisualization> Convert(IEnumerable<IAnalysisIssueFlow> flows)
         {
             if (!flows.Any())
             {
@@ -97,7 +97,7 @@ private IReadOnlyList<IAnalysisIssueFlowVisualization> Convert(IEnumerable<IAnal
             return convertedFlows;
         }
 
-        private IReadOnlyList<IAnalysisIssueLocationVisualization> Convert(IEnumerable<IAnalysisIssueLocation> locations)
+        private static IReadOnlyList<IAnalysisIssueLocationVisualization> Convert(IEnumerable<IAnalysisIssueLocation> locations)
         {
             var locationNumber = 1;