[1.0.X] Fixed #10028 -- Fixed a problem when ordering by related models.

Some results were inadvertently being excluded if we were ordering across a
nullable relation which itself ordering by a non-nullable relation.

Backport of r9916 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9917 bcc190cf-cafb-0310-a4f2-bffc1f526a37