Make sure the ContentType table exists before querying it in ContentTypeReference.__call__

[charettes]

This is a workaround for Django #24182.

[charettes]

Two things to do here:

• If the django_contenttype table is missing we should return None. It should be safe as no relations could be pointing to the table yet anyway.
• If the latest contenttypes migration is not applied we should fallback to manual querying,