A user with a plugin isn't the same as an empty password

[grooverdan]

Here if a plugin is set in the user table we don't list that as needed a password set.

[netsandbox]

@grooverdan @jmrenouard this fix is incomplete, as this wouldn't list users with an empty password and the 'mysql_native_password' or 'mysql_old_password' plugin.

Also, as the plugin is used since MySQL 5.5.7, I think there should be a MySQL Version dependent select, to find the correct users without a password for MySQL version < 5.5.7 and >= 5.5.7 (see http://dev.mysql.com/doc/refman/5.5/en/pluggable-authentication.html).

[grooverdan]

MySQL 5.7.6+ should probably use https://dev.mysql.com/doc/refman/5.7/en/show-create-user.html

authentication_string is the field added in 5.5.7 when plugins where added.