Symfony version(s) affected: 5.1.6 (same with 5.1.5)

Description
typehinting DateTime in a Form cause bugs...

If you dont put a value in a DateType in a Form, you have a Exception (Typed property App\Form\BugForm::$date must be an instance of DateTime, null used) with php 7.4 typed definition of the date (public \DateTime $date;) in place of a error message in the form.

How to reproduce

class BugForm
{

/**
 * @Assert\NotNull()
 */
public \DateTime $date;

}

class BugType extends AbstractType
{

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('date', DateType::class, ['widget' => 'single_text']);
}

public function configureOptions(OptionsResolver $resolver)
{
    $resolver->setDefaults(
        [
            'attr' => ['novalidate' => 'novalidate'],
            'data_class' => BugForm::class,
        ]
    );
}

}

To have the normal comportment, just dont use php 7.4 typed definition ("public $date" instead of "public \DateTime $date")