[1.2.X] Fixed #14859 -- ITSocialSecurityNumberField and ITVatNumberField...

[1.2.X] 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.  Backport of [14945].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14969 bcc190cf-cafb-0310-a4f2-bffc1f526a37