You can set the password to `''`. For example:

```
mysql> insert into users (user_id, username, passwd) values (426, 'testuser', '');
Query OK, 1 row affected (0.02 sec)
mysql> select * from users where user_id=426;
+---------+----------+--------+------+-----------+----------+
| user_id | username | passwd | salt | lastlogin | isactive |
+---------+----------+--------+------+-----------+----------+
|     426 | testuser |        | NULL | NULL      | NULL     |
+---------+----------+--------+------+-----------+----------+
1 row in set (0.00 sec)
```