Fixed #12546. Objects with a __len__ that returns 0 can now be serialized....

Fixed #12546. Objects with a __len__ that returns 0 can now be serialized. Thanks, casobn for the report and Alex Gaynor for the patch and tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12576 bcc190cf-cafb-0310-a4f2-bffc1f526a37