Fix optional fields in api

[hlohaus]

No description provided.

[github-actions]

Thanks for your contribution, @h Lohaus! This pull request addresses a good point and improves the code.

The changes to the ChatCompletionsConfig and ImageGenerationConfig classes are clean and make the optional fields more concise. Removing the Field(examples=[None]) is a great simplification.

The changes in g4f/client/__init__.py regarding safe_aclose look good, ensuring proper resource cleanup even if a generator doesn't have an aclose method.

The modifications to the async_generate and async_create_variation methods, especially the checks for hasattr(provider_handler, "create_async_generator") and hasattr(provider, "create_async_generator"), improve the robustness of the code by handling different provider types more gracefully. This change also resolves potential issues.

The change in publish-workflow.yaml to explicitly name the artifact is a good addition for clarity.

Overall, this pull request is well-written, easy to understand, and makes the code cleaner and more reliable. I approve these changes.