`$wpdb->get_col_length` always return false

On presence of HyperDB drop-in, `$wpdb::get_col_length()` always returns `false`.
This is because the `get_col_length()` method does an early-return if `$wpdb->is_mysql` is empty ([please see here](https://github.com/WordPress/wordpress-develop/blob/b071c28f8b6be06e464ccf582877e66c02859679/src/wp-includes/class-wpdb.php#L3370-L3373)).
the `is_mysql` property is never set in hyperdb, which causes this issue, while it is [hard-coded in `wpdb`](https://github.com/WordPress/wordpress-develop/blob/b071c28f8b6be06e464ccf582877e66c02859679/src/wp-includes/class-wpdb.php#L1949).