Fixed #17429 -- Ensured that `Meta.ordering=None` works the same if it were an empty list. Thanks to self[at]dicos[dot]ru for the report and to bigkevmcd for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17334 bcc190cf-cafb-0310-a4f2-bffc1f526a37