fix: use None instead of Reasoning() for reasoning parameter

The ResponsesAPIResponse.reasoning field expects Optional[Dict[str, Any]],
not a Reasoning object. Passing an empty Reasoning() object causes a mypy type error.

Fix: Pass None instead of Reasoning() since the field is optional and we're
not providing any reasoning data.

Fixes mypy error:
Argument "reasoning" to "ResponsesAPIResponse" has incompatible type
"Reasoning"; expected "dict[str, Any] | None"

litellm/responses/litellm_completion_transformation/transformation.py

@@ -1500,7 +1500,7 @@ class LiteLLMCompletionResponsesConfig:
             previous_response_id=getattr(
                 chat_completion_response, "previous_response_id", None
             ),
-            reasoning=dict(Reasoning()),
+            reasoning=None,
             status=LiteLLMCompletionResponsesConfig._map_chat_completion_finish_reason_to_responses_status(
                 finish_reason
             ),