fixed missing list input ref in query (#7375)

### What problem does this PR solve?

fixed missing list input ref in query

### Type of change

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

agent/component/keyword.py

 
     def _run(self, history, **kwargs):
         query = self.get_input()
-        query = str(query["content"][0]) if "content" in query else ""
+        if hasattr(query, "to_dict") and "content" in query:
+            query = ", ".join(map(str, query["content"].dropna()))
+        else:
+            query = str(query)
+
 
         chat_mdl = LLMBundle(self._canvas.get_tenant_id(), LLMType.CHAT, self._param.llm_id)
         ans = chat_mdl.chat(self._param.get_prompt(), [{"role": "user", "content": query}],