Add support to iframe chatbot (#3929)

### What problem does this PR solve?

#3909

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

agent/component/begin.py

@@ -26,6 +26,7 @@ class BeginParam(ComponentParamBase):
     def __init__(self):
         super().__init__()
         self.prologue = "Hi! I'm your smart assistant. What can I do for you?"
+        self.query = []
 
     def check(self):
         return True