Fixed #14499 -- ATSocialSecurityNumberField now responds to all EMPTY_VALUES...

Fixed #14499 -- ATSocialSecurityNumberField now responds to all EMPTY_VALUES correctly.  Also converted Austrian localflavor doctests to unittests.  We have always been at war with doctests.  Thanks to Idan Gazit for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14873 bcc190cf-cafb-0310-a4f2-bffc1f526a37