如何在Symfony表单约束中设置有效日期或空字段

how to validate date with constraints to allow date in format yyyy-mm-dd or leave date blank in this code?

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints as Assert;

class ArticleType extends AbstractType {

    public function getName() {
        return 'article';
    }

    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder
                ->add('date', 'date', array(
                    'label' => 'Date',
                    'input' => 'string',
                    'widget' => 'single_text',
                    'format' => 'yyyy-MM-dd',
                    'html5' => false,
                    'constraints' => array(
                        new Assert\NotBlank(),
                        new Assert\Date(),
                    ),
                ))

    }

}

Is it possible to validate without create custom validator?