Sokil\Mongo\Validator\NullValidator::validateField PHP Method

    public function validateField(Structure $document, $fieldName, array $params)
    {
        if (!$document->get($fieldName)) {
            return;
        }
        if (is_null($document->get($fieldName))) {
            return;
        }
        if (!isset($params['message'])) {
            $params['message'] = 'Field "' . $fieldName . '" must be null in model ' . get_called_class();
        }
        $document->addError($fieldName, $this->getName(), $params['message']);
    }