Fixed #16917 -- Don't try to use the model name for a ContentType's unicode...

Fixed #16917 -- Don't try to use the model name for a ContentType's unicode representation if the model no longer exists. Thanks Ivan Sagalaev for report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16895 bcc190cf-cafb-0310-a4f2-bffc1f526a37