.Net - Convert message AuthorName whitespace or empty string assignment to null (#6990)

### Motivation and Context
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Assist caller in avoiding an improper assignmemt of:

- `ChatMessageContent.AuthorRole`
- `StreamingChatMessageContent.AuthorRole`

### Description
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

In advertent assignment of an empty string (or whitespace) results in an
`HttpOperationException` when using the message as input to
`IChatCompletionService`.

While it may be ideal for caller to take care to _not_ assign such a
value, intercepting this state at the earliest possible moment reduces
debugging confusion.

> Stronger validation may not be appropriate as it may be model
specific.

### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->
- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: crickman

dotnet/src/SemanticKernel.Abstractions/Contents/ChatMessageContent.cs

@@ -20,7 +20,11 @@ public class ChatMessageContent : KernelContent
     /// </summary>
     [Experimental("SKEXP0001")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-    public string? AuthorName { get; set; }
+    public string? AuthorName
+    {
+        get => this._authorName;
+        set => this._authorName = string.IsNullOrWhiteSpace(value) ? null : value;
+    }
 
     /// <summary>
     /// Role of the author of the message
@@ -171,4 +175,5 @@ public override string ToString()
 
     private ChatMessageContentItemCollection? _items;
     private Encoding _encoding;
+    private string? _authorName;
 }

dotnet/src/SemanticKernel.Abstractions/Contents/StreamingChatMessageContent.cs

@@ -64,7 +64,11 @@ public StreamingKernelContentItemCollection Items
     /// </summary>
     [Experimental("SKEXP0001")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-    public string? AuthorName { get; set; }
+    public string? AuthorName
+    {
+        get => this._authorName;
+        set => this._authorName = string.IsNullOrWhiteSpace(value) ? null : value;
+    }
 
     /// <summary>
     /// Role of the author of the message
@@ -126,4 +130,5 @@ public StreamingChatMessageContent(AuthorRole? role, string? content, object? in
 
     private StreamingKernelContentItemCollection? _items;
     private Encoding _encoding;
+    private string? _authorName;
 }

dotnet/src/SemanticKernel.UnitTests/Contents/ChatMessageContentTests.cs

@@ -128,6 +128,25 @@ public void ContentPropertyGetterShouldReturnContentOfTheFirstTextContentItem()
         Assert.Equal("fake-content-1", sut.Content);
     }
 
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData(" ")]
+    [InlineData("\t")]
+    [InlineData("\n")]
+    [InlineData("\r\n")]
+    public void ContentPropertySetterShouldConvertEmptyOrWhitespaceAuthorNameToNull(string? authorName)
+    {
+        // Arrange
+        var message = new ChatMessageContent(AuthorRole.User, content: null)
+        {
+            AuthorName = authorName
+        };
+
+        // Assert
+        Assert.Null(message.AuthorName);
+    }
+
     [Fact]
     public void ItShouldBePossibleToSetAndGetEncodingEvenIfThereAreNoItems()
     {

dotnet/src/SemanticKernel.UnitTests/Contents/StreamingChatMessageContentTests.cs

@@ -118,6 +118,25 @@ public void ContentPropertyGetterShouldReturnContentOfTheFirstTextContentItem()
         Assert.Equal("fake-content-1", sut.Content);
     }
 
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData(" ")]
+    [InlineData("\t")]
+    [InlineData("\n")]
+    [InlineData("\r\n")]
+    public void ContentPropertySetterShouldConvertEmptyOrWhitespaceAuthorNameToNull(string? authorName)
+    {
+        // Arrange
+        var message = new StreamingChatMessageContent(AuthorRole.User, content: null)
+        {
+            AuthorName = authorName
+        };
+
+        // Assert
+        Assert.Null(message.AuthorName);
+    }
+
     [Fact]
     public void ItShouldBePossibleToSetAndGetEncodingEvenIfThereAreNoItems()
     {