xPDOQuery check for NULL

where=`{"close_date":null}`

<?php
$users = $modx->getCollection('modUser', $modx->fromJSON('{"remote_key":null}'));
foreach ($users as $user) {
    var_dump($user->toArray());
}

    public function where($criteria) {
        if (is_array($criteria)) {
            foreach ($criteria as $col => $v) {
                $this->condition(array($col => $v));
            }
        } else {
            $this->condition($criteria);
        }
    }
    protected function condition($criteria) {
        $this->_where[] = $criteria;
        echo var_dump( $this->_where);
    }

SELECT * FROM `modx_jobs`
WHERE `close_date` = ""
ORDER BY `publishedon` DESC
LIMIT 3