MariaDB doesn't set authentication_string during installation
Unlike newer versions of MySQL (5.7+), MariaDB doesn't seem to use the authentication_string of the user table for normal passwords when using the default plugin (i.e. mysql_native_password). Instead, the passwords are found in the password column, like in older versions of MySQL. 

However, you can get something into the authentication_string if you create a user using e.g. the pam plugin:

CREATE USER u1@localhost IDENTIFIED VIA pam USING 'mariadb';


(authentiation_string now contains 'mariadb' in plaintext for this user, though this is not the password, just a reference to a PAM configuration file. With PAM the password is no longer stored in MariaDB.)