Bugfix to handle creation of nullables ManyToManyField

When upgrading from 1.0 version to 1.1, I found a bug that was enabling creation of a lot of `ManyToManyFiedls` with null flag assigned as `True`. This bug affected drastically my tests execution time and also made a few one breaks because they didn't expected the objects from m2m relationship to be created.

This pull request fixes it by refactoring a little the logic to determine m2m values.