fix(providers): normalize empty reasoning_content to None at provider level

PR #947 fixed the consumer side (context.py) but the root cause is at
the provider level — getattr returns "" (empty string) instead of None
when reasoning_content is empty. This causes DeepSeek API to reject the
request with "Missing reasoning_content field" error.

`"" or None` evaluates to None, preventing empty strings from
propagating downstream.

Fixes #946

nanobot/providers/litellm_provider.py

@@ -257,7 +257,7 @@ class LiteLLMProvider(LLMProvider):
                 "total_tokens": response.usage.total_tokens,
             }
         
-        reasoning_content = getattr(message, "reasoning_content", None)
+        reasoning_content = getattr(message, "reasoning_content", None) or None
         
         return LLMResponse(
             content=message.content,