Re-adding check for None to clean() (#80)

[NinjaDero]

This is basically just the removed check for None and isinstance(queryset, QuerySet) back in place.
If value is None, it can not be iterated and throws an error when handling with pages, articles or any app using this module in Django CMS.

I also bumped the version, to make writing pip requirements easier, like:

django-sortedm2m!=1.3.0,>=1.2.2

to skip version 1.3.0.dev1.

[gregmuellegger]

Yes, that change makes sense. Sorry that the removal of this check slipped through the review. What do you think about testing for an iterable instead of a queryset? We actually don't require it to be a queryset.

[NinjaDero]

Sure, I just went with the approach of least damage and added the line close to what it was before.
I went with checking with hasattr() instead of iter(), because iter will fall back to __getitem__ if __iter__ is not present.
Is that alright? What are possible iterables that could fail?

[gregmuellegger]

Just released 1.3.2, containing your fix. Thanks for the effort!
https://pypi.python.org/pypi/django-sortedm2m