Fix:HTTP request component failed to retrieve the corresponding value (#9399)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/9385
Based on my understanding, I think checking empty string is fine

### Type of change

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

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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

agent/component/invoke.py

     def _invoke(self, **kwargs):
         args = {}
         for para in self._param.variables:
-            if para.get("value") is not None:
+            if para.get("value"):
                 args[para["key"]] = para["value"]
             else:
                 args[para["key"]] = self._canvas.get_variable_value(para["ref"])
         assert False, self.output()
 
     def thoughts(self) -> str:
-        return "Waiting for the server respond..."
+        return "Waiting for the server respond..."