0.6 breaks default blank value

[jonashaag]

EDIT: This just happened in production as well, so it's not a model-mommy/test problem. Production: None is the default database value of a separated values field, when it should be ''.

With Django < 2 and model-mommy, default value for the separated values field is incorrectly None (rather than ''). Now model-mommy doesn't actually support separated values field, but if it has blank=True it used to work anyways. I tried to pinpoint the exact reason but I'm not sure where it is.

To reproduce:

from django.db import models
from separatedvaluesfield.models import SeparatedValuesField

class TestModel(models.Model):
    f = SeparatedValuesField(max_length=255, blank=True)

model_mommy.mommy.make('testapp.TestModel')

With 0.5 this resulted in the following insert query:

INSERT INTO "testapp_testmodel" ("f") VALUES (?) ['']

With 0.6 this results in the following insert query:

INSERT INTO "testapp_testmodel" ("f") VALUES (?) [None]

The None value violates the separated value field's NOT NULL constraint, making the insertion fail.