.Net: Stream response should not skip white space (microsoft#2922)

### Motivation and Context
The current Azure OpenAI streaming implementation skips white space (ie.
space character token ID 220). All non-empty tokens should be streamed,
including space characters. This is rare as most tokens are prefixed
with a space by default, but still an issue in some cases (such as
formatted markdown, code, etc).

Fixes microsoft#2921 

### Description
Modify the streaming logic to check for null or empty instead of null,
empty and white space.

### 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 😄

Co-authored-by: Roger Barreto <19890735+RogerBarreto@users.noreply.github.com>

dotnet/src/Connectors/Connectors.AI.OpenAI/AzureSdk/ChatStreamingResult.cs

@@ -47,7 +47,7 @@ public async IAsyncEnumerable<ChatMessageBase> GetStreamingChatMessageAsync([Enu
     {
         await foreach (var message in this._choice.GetMessageStreaming(cancellationToken))
         {
-            if (!string.IsNullOrWhiteSpace(message.Content))
+            if (message.Content is { Length: > 0 })
             {
                 yield return new SKChatMessage(message);
             }
@@ -65,9 +65,9 @@ public async IAsyncEnumerable<string> GetCompletionStreamingAsync([EnumeratorCan
     {
         await foreach (var result in this.GetStreamingChatMessageAsync(cancellationToken).ConfigureAwait(false))
         {
-            if (!string.IsNullOrWhiteSpace(result.Content))
+            if (result.Content is string content and { Length: > 0 })
             {
-                yield return result.Content;
+                yield return content;
             }
         }
     }

dotnet/src/Connectors/Connectors.AI.OpenAI/ChatCompletionWithData/ChatWithDataStreamingResult.cs

@@ -44,7 +44,7 @@ public async IAsyncEnumerable<ChatMessageBase> GetStreamingChatMessageAsync([Enu
     {
         var message = await this.GetChatMessageAsync(cancellationToken).ConfigureAwait(false);
 
-        if (!string.IsNullOrWhiteSpace(message.Content))
+        if (message.Content is { Length: > 0 })
         {
             yield return message;
         }
@@ -54,9 +54,9 @@ public async IAsyncEnumerable<string> GetCompletionStreamingAsync([EnumeratorCan
     {
         await foreach (var result in this.GetStreamingChatMessageAsync(cancellationToken))
         {
-            if (!string.IsNullOrWhiteSpace(result.Content))
+            if (result.Content is string content and { Length: > 0 })
             {
-                yield return result.Content;
+                yield return content;
             }
         }
     }