[1.2.X] Fixed #14499 -- ATSocialSecurityNumberField now responds to all...

[1.2.X] 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.  Backport of [14873].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14875 bcc190cf-cafb-0310-a4f2-bffc1f526a37