Code refactor. (#4291)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

agent/component/answer.py

@@ -16,6 +16,7 @@
 import random
 from abc import ABC
 from functools import partial
+from typing import Tuple, Union
 
 import pandas as pd
 
@@ -76,4 +77,13 @@ class Answer(ComponentBase, ABC):
     def set_exception(self, e):
         self.exception = e
 
+    def output(self, allow_partial=True) -> Tuple[str, Union[pd.DataFrame, partial]]:
+        if allow_partial:
+            return super.output()
+
+        for r, c in self._canvas.history[::-1]:
+            if r == "user":
+                return self._param.output_var_name, pd.DataFrame([{"content": c}])
+
+        self._param.output_var_name, pd.DataFrame([])