fix: always include output in function call result message (#2414)

This follows the official OpenAI API schema

Author: Hedrekao

python/packages/core/agent_framework/openai/_responses_client.py

@@ -518,9 +518,8 @@ def _openai_content_parser(
                 args: dict[str, Any] = {
                     "call_id": content.call_id,
                     "type": "function_call_output",
+                    "output": prepare_function_call_results(content.result),
                 }
-                if content.result:
-                    args["output"] = prepare_function_call_results(content.result)
                 return args
             case FunctionApprovalRequestContent():
                 return {