Fix another potential bug concerning UserData object with no data in …

…them

There are no calls to UserData.__call__ that also pass a value for  that I can find.
But if someone were to do this, it would suffer from the same bug: a UserData instance with no data in it would evaluate to False
and thus the  would be used, which I thik is not what is intended when there is a reference UserData passed explicitly

flexbe_core/src/flexbe_core/core/user_data.py

@@ -65,7 +65,7 @@ def __setattr__(self, key, value):
         self[key] = value
 
     def __call__(self, reference=None, add_from=None, update_from=None, remove_key=None):
-        self._reference = reference or self._reference
+        self._reference = reference if reference is not None else self._reference
         if isinstance(add_from, UserData):
             for key, value in add_from._data.items():
                 if key not in self._data: