Create user: no password for ldap

At the moment create user has a problem with LDAP auth method: it doesn't require a password, but our users always have to enter with password '' for it. We would like to support something like:

CREATE USER user [ [ WITH ] PASSWORD 'password' ] [ USING ldap | md5 | chap-sha1 ]

If password is not set, then its default value is empty string. Each auth method should support password length check. So, if the password is absent for ldap, its length check allows us this empty string. Other methods should fail with password too short if the password is not defined.

As a result, this command would be correct:

CREATE USER alice USING ldap

The default auth method is still chap-sha1.