Fixed #14859 -- ITSocialSecurityNumberField and ITVatNumberField didn't handle all EMPTY_VALUES gracefully.  Also converted the Italias localflavor doctests into unittests.  We have always been at war with doctests.  Thanks to Idan Gazit.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14945 bcc190cf-cafb-0310-a4f2-bffc1f526a37