Ubuntu by default has no root password, and you can also configure Debian in that way if you choose the advanced installation option, and decide not to use the root password.

But what to do if later you need the root user? well you just need to assign a password to the root user and then you will be able to switch to root user.

sudo passwd

Type a good strong password twice, and you are done.

su -

Type the root password and voila! you are logged as root, you know you have to be carefull on what you do being root right?