Include the depends of the getter function when depending on a Function field
Since c47b3642f1a6, the getter of a Function field may be called if there is no value set for it on the instance. In the on_change context, this may set the missing fields to None if they were not defined in the depends of the method called. In addition to compute a wrong value, those fields are then returned as modified.
Closes #13735 (closed)