fix mutiple retrieval component content (#1997)

### What problem does this PR solve?

### Type of change

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

agent/component/base.py

             if self.component_name.lower() == "generate" and self.get_component_name(u) == "retrieval":
                 o = self._canvas.get_component(u)["obj"].output(allow_partial=False)[1]
                 if o is not None:
+                    if not "".join(o["content"]):
+                        continue
                     upstream_outs.append(o)
                     continue
             if u not in self._canvas.get_component(self._id)["upstream"]: continue

agent/component/generate.py

 
     def stream_output(self, chat_mdl, prompt, retrieval_res):
         res = None
-        if "empty_response" in retrieval_res.columns and "\n- ".join(retrieval_res["content"]):
-            res = {"content": "\n- ".join(retrieval_res["content"]), "reference": []}
+        if "empty_response" in retrieval_res.columns and not "\n- ".join(retrieval_res["content"]):
+            res = {"content": "\n- ".join(retrieval_res["empty_response"]), "reference": []}
             yield res
             self.set_output(res)
             return

agent/component/retrieval.py

                                         aggs=False, rerank_mdl=rerank_mdl)
 
         if not kbinfos["chunks"]:
-            df = Retrieval.be_output(self._param.empty_response)
-            df["empty_response"] = True
+            df = Retrieval.be_output("")
+            df["empty_response"] = self._param.empty_response
             return df
 
         df = pd.DataFrame(kbinfos["chunks"])