Monkeypatch ReferenceProperty to suit our needs
Monkeypatch ReferenceProperty to suit our needs

Since ReferenceProperties make_value_from_form does not call the base
make_value_from_form method, there is no way to set a reference to
None. Address this by converting '' to None.

SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange,SRabbelier/Melange