Throw warning when applies_to is empty (#1245)

* Return null if applies to is empty and emit warning

* Update docs

* Add test

* Fix error

* Fix tests

Author: theletterf

docs/syntax/applies.md

@@ -47,22 +47,22 @@ discontinued 9.2.0
 all
 ```
 
-`all` and empty string mean generally available for all active versions
+`all` means generally available for all active versions
 
 ```yaml
 applies_to:
   serverless: all
 ```
 
-`all` and empty string can also be specified at a version level
+`all` can also be specified at a version level
 
 ```yaml
 applies_to:
   stack: beta all
   serverless: beta
 ```
 
-Both are equivalent, note `all` just means we won't be rendering the version portion in the html.
+Note `all` just means we won't be rendering the version portion in the HTML.
 
 
 ## Structured model

src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs

@@ -122,10 +122,17 @@ public class ApplicableToConverter : IYamlTypeConverter
 
 	public object? ReadYaml(IParser parser, Type type, ObjectDeserializer rootDeserializer)
 	{
+		var warnings = new List<string>();
+		var applicableTo = new ApplicableTo();
+
 		if (parser.TryConsume<Scalar>(out var value))
 		{
 			if (string.IsNullOrWhiteSpace(value.Value))
-				return ApplicableTo.All;
+			{
+				warnings.Add("The 'applies_to' field is present but empty. No applicability will be assumed.");
+				return null;
+			}
+
 			if (string.Equals(value.Value, "all", StringComparison.InvariantCultureIgnoreCase))
 				return ApplicableTo.All;
 		}
@@ -134,10 +141,6 @@ public class ApplicableToConverter : IYamlTypeConverter
 		if (deserialized is not Dictionary<object, object?> { Count: > 0 } dictionary)
 			return null;
 
-
-		var applicableTo = new ApplicableTo();
-		var warnings = new List<string>();
-
 		var keys = dictionary.Keys.OfType<string>().ToArray();
 		var oldStyleKeys = keys.Where(k => k.StartsWith(':')).ToList();
 		if (oldStyleKeys.Count > 0)

tests/authoring/Applicability/AppliesToFrontMatter.fs

@@ -23,7 +23,7 @@ applies_to:
 """
     [<Fact>]
     let ``apply matches expected`` () =
-        markdown |> appliesTo ApplicableTo.All
+        markdown |> appliesTo (Unchecked.defaultof<ApplicableTo>)
 
 type ``apply default to top level arguments`` () =
     static let markdown = frontMatter """
@@ -186,3 +186,11 @@ applies_to:
             Stack=AppliesCollection.op_Explicit "ga 9.1.0",
             Product=AppliesCollection.op_Explicit "coming 9.5, discontinued 9.7"
         ))
+
+type ``parses empty applies_to as null`` () =
+    static let markdown = frontMatter """
+applies_to:
+"""
+    [<Fact>]
+    let ``does not render label`` () =
+        markdown |> appliesTo (Unchecked.defaultof<ApplicableTo>)