[1.0.X] This fixes a group of problems in the SQL created by QuerySet.exclude()

when used in a few situations where NULL results can appear.

Fixed #8921 (the only ticket I know of that noticed any of these).

Backport of r9590 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9591 bcc190cf-cafb-0310-a4f2-bffc1f526a37