Fix generate param empty_response (#2010)

### What problem does this PR solve?

### Type of change

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

agent/component/generate.py

@@ -67,7 +67,8 @@ class Generate(ComponentBase):
 
     def set_cite(self, retrieval_res, answer):
         retrieval_res.dropna(subset=["vector", "content_ltks"], inplace=True)
-        retrieval_res["empty_response"].fillna("", inplace=True)
+        if "empty_response" in retrieval_res.columns:
+            retrieval_res["empty_response"].fillna("", inplace=True)
         answer, idx = retrievaler.insert_citations(answer, [ck["content_ltks"] for _, ck in retrieval_res.iterrows()],
                                                    [ck["vector"] for _, ck in retrieval_res.iterrows()],
                                                    LLMBundle(self._canvas.get_tenant_id(), LLMType.EMBEDDING,