Fix 'NoneType' object is not iterable when using __in with empty list.

[wsot]

Problem case:
model.objects.filter(<field>__in=[]) produces a 'NoneType' object is not iterable exception.

Cause:
get_prep_lookup in _NetAddressField returns None if value is falsy.

Change:
Remove early return in get_prep_lookup. Superclass's get_prep_lookup will be called with the value instead.

Added unit test for __in empty list case.
An alternative (neater?) unit test would be simply performing a self.qs.filter(field__in=[]) and checking it returns an empty query set.

[jimfunk]

Thanks, but the unit tests are failing on Django 1.9.

[jimfunk]

Seems that a broken Travis config kept me from seeing that these tests also fail in master. I will have to check into that