Add PHP 8.4 compatibility

[Slamdunk]

Fixes 2 issues:

CuyZ\Valinor\Mapper\Tree\Shell::child(): Implicitly marking parameter $attributes as nullable is deprecated, the explicit nullable type must be used instead
CuyZ\Valinor\Type\Types\ClassStringType::__construct(): Implicitly marking parameter $subType as nullable is deprecated, the explicit nullable type must be used instead

See https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

This PR doesn't add PHP 8.4 support, it just makes it runnable in PHP 8.4 without errors.

[romm]

Thank you @Slamdunk!