git: Handle None values in ChatMessage content

[Lucaz0619]

What does this PR do?

Fixes None values in ChatMessage content.
This PR aligns the ChatMessage model with the OpenAI API spec by allowing content to be either a string or null. It also updates the non_streaming_completion method to handle None values, preventing potential TypeError during message concatenation.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• [v] Did you read the contributor guideline, Pull Request section?
• [v] Did you make sure to update the docs?
• Did you write any new necessary tests?

💡 How Does This PR Impact Users?

✅ Improved OpenAI API Compatibility:
Users integrating with the OpenAI API will no longer face errors when the content field is null (which is always the case when OpenAI returns a function call). This ensures smooth interoperability, especially when handling function calls.
✅ Flexible Function Call Handling:
When OpenAI returns function calls with null content, users can now decide whether to handle these functions directly in the predict function, providing greater flexibility in managing LLM responses.

PR review

Focus on the changes to the ChatMessage model and non_streaming_completion method.

Did you have fun?

Absolutely! 🙃

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89%. Comparing base (54f1430) to head (75d61da).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #422   +/-   ##
===================================
- Coverage    89%    89%   -0%     
===================================
  Files        31     31           
  Lines      2021   2023    +2     
===================================
+ Hits       1807   1808    +1     
- Misses      214    215    +1     

[aniketmaurya]

Looks great! thanks for the fix 🚀