Fix: keep behavior consistent for converse_with_chat_assistant (#12190)

### What problem does this PR solve?

Keep behavior consistent for converse_with_chat_assistant. #12188

```markdown
2025-12-25 10:02:17,718 ERROR    11674 OpenAI async completion
openai.BadRequestError: Error code: 400 - {'error': {'code': '1213', 'message': '未正常接收到prompt参数。'}}
2025-12-25 10:02:17,718 ERROR    11674 async base giving up: **ERROR**: INVALID_REQUEST - Error code: 400 - {'error': {'code': '1213', 'message': '未正常接收到prompt参数。'}}

```

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/conversation_service.py

@@ -116,6 +116,16 @@ async def async_completion(tenant_id, chat_id, question, name="New session", ses
                                     ensure_ascii=False) + "\n\n"
             yield "data:" + json.dumps({"code": 0, "message": "", "data": True}, ensure_ascii=False) + "\n\n"
             return
+        else:
+            answer = {
+                "answer": conv["message"][0]["content"],
+                "reference": {},
+                "audio_binary": None,
+                "id": None,
+                "session_id": session_id
+            }
+            yield answer
+            return
 
     conv = ConversationService.query(id=session_id, dialog_id=chat_id)
     if not conv: