Describe the bug
The extension method ChatCompletionExtensions.GenerateMessageStreamAsync() returns an unexpected null as last message.

To Reproduce
Steps to reproduce the behavior:

1. Consider this sample
2. Change the code to Console.Write(message ?? "#NULL#"); to highlight any null value returned

Expected behavior
GenerateMessageStreamAsync() doesn't return any null string.

Platform

• OS: Mac
• IDE: Rider
• Language: C#
• Source: NuGet Microsoft.SemanticKernel version="0.24.230918.1-preview