PHP 8.4 deprecation: Implicitly marking parameter as nullable

Problem/Motivation

Drupal 11.1 is compatible with PHP 8.4 but the Subrequests module currently triggers a deprecation warning:

Deprecated: Drupal\subrequests\Normalizer\JsonBlueprintDenormalizer::setValidator(): Implicitly marking parameter $validator as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/subrequests/src/Normalizer/JsonBlueprintDenormalizer.php on line 56