Suggestion: convert null content to empty string for Azure compatibility

[HappyLiang12]

When using CC Switch to translate Responses API → Chat Completions with Azure OpenAI as the backend, requests with "content": null get rejected by Azure.

Azure strictly requires content to be a string, while the Responses API allows null for tool-call-only messages. Other providers like Alicloud are more lenient and accept null.

Would it make sense for CC Switch to convert null → "" during translation? This would improve compatibility with strict providers without breaking lenient ones.

Version: v5.10.4 (Windows x64)

[SaladDay]

Merged into main in 8913e7c1. Empty Chat message content now uses "" instead of null; tool payloads are preserved.