smb: client: fix regression with guest option

JIRA: https://issues.redhat.com/browse/RHEL-83858

commit fc99045effa81fdf509c2a97cbb7e6e8f2fd4443
Author: Paulo Alcantara <pc@manguebit.com>
Date:   Wed Mar 12 10:51:31 2025 -0300

    smb: client: fix regression with guest option

    When mounting a CIFS share with 'guest' mount option, mount.cifs(8)
    will set empty password= and password2= options.  Currently we only
    handle empty strings from user= and password= options, so the mount
    will fail with

            cifs: Bad value for 'password2'

    Fix this by handling empty string from password2= option as well.

    Link: https://bbs.archlinux.org/viewtopic.php?id=303927


Reported-by: Adam Williamson <awilliam@redhat.com>
    Closes: https://lore.kernel.org/r/83c00b5fea81c07f6897a5dd3ef50fd3b290f56c.camel@redhat.com


    Fixes: 35f834265e0d ("smb3: fix broken reconnect when password changing on the server by allowing password rotation")
    Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Signed-off-by: Paulo Alcantara <paalcant@redhat.com>