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: 35f83426 ("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>