[1.0.X] Improved table join handling for comparisons against NULL.

This fixes a broad class of bugs involving filters that look for missing
related models and fields. Most of them don't seem to have been reported
(the added tests cover the root cause). The exception is that this has
also fixed #9968.

Backport of r9979 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9980 bcc190cf-cafb-0310-a4f2-bffc1f526a37