If for some reason you have lost your /dev/null you can easily recreate it by moving a file to /dev/null as root. Another way is to just recreate the soft link (as root again):
devlinks
But if you have messed up the original device in /devices proceed as follows:
mknod /devices/pseudo/mm@0:null c 13 2
chown root:sys /devices/pseudo/mm@0:null
chmod 666 /devices/pseudo/mm@0:null
cd /dev
ln -s ../devices/pseudo/mm@0:null null