changed default for an unspecified model to None if ZEBRA_ENABLE_APP is off.
changed default for an unspecified model to None if ZEBRA_ENABLE_APP is off.

gumob/django-stripe,blag/django-zebra,blag/django-zebra,blag/django-zebra,amjoconn/django-zebra,GoodCloud/django-zebra,gumob/django-stripe,GoodCloud/django-zebra,gumob/django-stripe,amjoconn/django-zebra,amjoconn/django-zebra