Fixing -bash: /dev/null: Permission denied

 This should fix the issue (as root):

rm /dev/null
mknod /dev/null c 1 3
chmod 666 /dev/null

What these commands are doing:

rm is removing the bogus file that has been created because the expected one was missing;
mknod is creating a character device named /dev/null with the appropriate major and minor numbers for a Linux kernel;
chmod is setting the permissions for all users to be able to read and write to /dev/null.