Fixed #14423 -- corrected incorrect SQL being generated when a nullable,...

Fixed #14423 -- corrected incorrect SQL being generated when a nullable, inherited field was used in an exclude.  Thanks to PhiR_42 for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14600 bcc190cf-cafb-0310-a4f2-bffc1f526a37