NotNullConstraintValidator.php

<?php

namespace Drupal\Core\Validation\Plugin\Validation\Constraint;

use Drupal\Core\Config\Schema\ArrayElement;
use Drupal\Core\TypedData\ComplexDataInterface;
use Drupal\Core\TypedData\ListInterface;
use Drupal\Core\TypedData\Validation\TypedDataAwareValidatorTrait;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\Constraints\NotNullValidator;

/**
 * NotNull constraint validator.
 *
 * Overrides the symfony validator to handle empty Typed Data structures.
 */
class NotNullConstraintValidator extends NotNullValidator {
    use TypedDataAwareValidatorTrait;
    
    /**
     * {@inheritdoc}
     */
    public function validate($value, Constraint $constraint) : void {
        $typed_data = $this->getTypedData();
        // TRICKY: the Mapping and Sequence data types both extend ArrayElement
        // (which implements ComplexDataInterface), but configuration schema sees a
        // substantial difference between an empty sequence/mapping and NULL. So we
        // want to make sure we don't treat an empty array as NULL.
        if (($typed_data instanceof ListInterface || $typed_data instanceof ComplexDataInterface) && !$typed_data instanceof ArrayElement && $typed_data->isEmpty()) {
            $value = NULL;
        }
        parent::validate($value, $constraint);
    }

}