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).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9590 bcc190cf-cafb-0310-a4f2-bffc1f526a37