fix: empty or contains only empty strings. (#2347)

### What problem does this PR solve?
the setting was kept empty for Empty_response. In expectation, this case
should get a response from the LLM if can't find the references from the
knowledgebase.

### Type of change

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


![image](https://github.com/user-attachments/assets/9c382b1d-40f6-43b0-848c-fa6863f9a253)

![image](https://github.com/user-attachments/assets/032d2001-97a2-4faa-91bf-c9c57caf2070)

Co-authored-by: Theta Wang (ncu) <chunshan.connect@gmail.com>

agent/component/retrieval.py

 
         if not kbinfos["chunks"]:
             df = Retrieval.be_output("")
-            df["empty_response"] = self._param.empty_response
+            if self._param.empty_response and self._param.empty_response.strip():
+                df["empty_response"] = self._param.empty_response
             return df
 
         df = pd.DataFrame(kbinfos["chunks"])