[Migrations] `null=True, unique=True` Raises `django.db.utils.IntegrityError: UNIQUE constraint failed` when running migration

[jjorissen52]

To reproduce, make sure some rows already exist for MyModel, then add a PhoneField to it.

class MyModel(models.Model):
    -pass
    +phone = PhoneField(null=True, blank=True, unique=True)

python manage.py makemigrations
python manage.py migrate
# django.db.utils.IntegrityError: UNIQUE constraint failed: new__myapp_mymodel.phone