Fixed #14354 -- Normalized the handling of empty/null passwords in...

Fixed #14354 -- Normalized the handling of empty/null passwords in contrib.auth. This also updates the createsuperuser command to be more testable, and migrates some auth doctests. Thanks to berryp for the report, and Laurent Luce for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14053 bcc190cf-cafb-0310-a4f2-bffc1f526a37