Cleanup handling of NULL value passed for a mount option

Allow blank user= and ip= mount option. Also clean up redundant
checks for NULL values since the token parser will not actually
match mount options with NULL values unless explicitly specified.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reported-by: Chris Clayton <chris2553@googlemail.com>
Acked-by: Jeff Layton <jlayton@samba.org>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>