Regression in 1.3

Hello,
I'm getting a `TypeError NoneType object is not iterable` when saving an object in the admin.
I think it's related to the line above where the check for None was removed.
https://github.com/gregmuellegger/django-sortedm2m/commit/428a8a85c0e7fad42e6d89db4d3a0ab10a508017#commitcomment-17613670
