Fixed #9498 -- Handle a formset correctly when the foreign key is not available (for now).

This case pops up with generic foreign key inlines after [9297]. Added tests
to handle future regressions with generic foreign key inlines in the admin.

Thanks markus and danielr for patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9412 bcc190cf-cafb-0310-a4f2-bffc1f526a37