[1.2.X] Fixed #14423 -- corrected incorrect SQL being generated when a...

[1.2.X] Fixed #14423 -- corrected incorrect SQL being generated when a nullable, inherited field was used in an exclude.  Thanks to PhiR_42 for the patch. Backport of [14600].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14601 bcc190cf-cafb-0310-a4f2-bffc1f526a37