add OUTPUT_URL=null support

the patch add support for "OUTPUT_URL=null" and fix my situation below. maybe there are other situations which user will want to set "null" OUTPUT_URL.

my /etc/rear/local.conf is setup like below base on the advice from email list:
BACKUP=NETFS
BACKUP_URL=iso://backup/
OUTPUT=ISO
OUTPUT_URL=nfs://10.1.1.1/share/nfs/

the archived file will first goto /tmp, then create iso to /var/lib/rear/output, and finally copy iso to nfs share. if the iso file is large, it will consume  huge space at local server. so I tried to tune "ISO_DIR" to remote server.
then I don't need OUTPUT_URL anymore. but rear won't let me set it to blank, so I add 'null' method for it.