Fix：The OpenAI-Compatible Agent API returns an incorrect message (#8177)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8175

### Type of change

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

agent/component/answer.py

@@ -64,14 +64,17 @@ class Answer(ComponentBase, ABC):
             for ii, row in stream.iterrows():
                 answer += row.to_dict()["content"]
                 yield {"content": answer}
-        else:
+        elif stream is not None:
             for st in stream():
                 res = st
                 yield st
-        if self._param.post_answers:
+        if self._param.post_answers and res:
             res["content"] += random.choice(self._param.post_answers)
             yield res
 
+        if res is None:
+            res = {"content": ""}
+
         self.set_output(res)
 
     def set_exception(self, e):