Fix:valueERROR when file is optional but not exist value (#9414)

### What problem does this PR solve?

when begin component has optional file but not exist , it rase error

### Type of change

- [x] Bug Fix

Co-authored-by: Popmio <zhengyihao036@gamil.com>

agent/component/begin.py

     def _invoke(self, **kwargs):
         for k, v in kwargs.get("inputs", {}).items():
             if isinstance(v, dict) and v.get("type", "").lower().find("file") >=0:
-                v = self._canvas.get_files([v["value"]])
+                if v.get("optional") and v.get("value", None) is None:
+                    v = None
+                else:
+                    v = self._canvas.get_files([v["value"]])
             else:
                 v = v.get("value")
             self.set_output(k, v)