Refactor create query to fix meta.ignore_none_field = True

[ADR-007]

meta.ignore_none_field now means: Ignore default None values but save None if it was set explicitly:

MyModel().save()  # ignore Nones

MyModel(my_field=None).save()   # ignore Nones but not for my_field

MyModel(my_field=None).update(key)  # partial update with my_field is None