To create a user with an empty password, the user must first exist in /etc/passwd and /etc/shadow.
The hash of the empty password is U6aMy0wojraho (0 is a zero...) and it should be entered in the second field in the row in /etc/shadow, for instance with
sed -i 's/^root:[^:]:/root:U6aMy0wojraho:/' /etc/shadow
or usermod, or you create a new user with
useradd myuser -p U6aMy0wojraho
To have a terminal (for instance the terminal on the serial port) automatically login without password, add
after the ttyS0 getty entry in /etc/inittab or in the file /etc/init/ttyS0.conf on ubuntu.