Fix:when stream=false，new message without sessionid does no (#8078)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/8070

### Type of change

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

api/db/services/conversation_service.py

@@ -90,6 +90,7 @@ def completion(tenant_id, chat_id, question, name="New session", session_id=None
             "user_id": kwargs.get("user_id", "")
         }
         ConversationService.save(**conv)
+        if stream:
             yield "data:" + json.dumps({"code": 0, "message": "",
                                         "data": {
                                             "answer": conv["message"][0]["content"],