public static function nulls(array $data, $keys = null)
    {
        if ($keys === null) {
            $keys = array_keys($data);
        }

        if (!Arrays::isArray($keys)) {
            $keys = array($keys);
        }

        foreach ($keys as $key) {
            if (array_key_exists($key, $data) && empty($data[$key])) {
                $data[$key] = null;
            }
        }

        return $data;
    }