Meaning of `cat /dev/null > file`

The command will output the data from device /dev/null to the given file (mailbox of the root account). Since /dev/null responds just with end-of-file when reading from it nothing will be written to the file, but with the redirection > the shell will have cleared the file already. Actually this is equivalent to writing just

> /var/spool/mail/root

(i.e., the same without cat or /dev/null).

cat /dev/null > /var/spool/mail/root truncates /var/spool/mail/root

Alternative is > /var/spool/mail/root