Fixed #7181 -- when ordering by a potentially NULL field, use a left-outer join

so that the ordering doesn't accidentally restrict the result set.

(Ironically, one existing test actually showed this problem, but I was too
dumb to notice the result was incorrect.)


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7761 bcc190cf-cafb-0310-a4f2-bffc1f526a37