Fixed corner-case bug in formtools wizard.py and preview.py, in case of a None...

Fixed corner-case bug in formtools wizard.py and preview.py, in case of a None value -- thanks, Honza

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7277 bcc190cf-cafb-0310-a4f2-bffc1f526a37