Declare nullable parameter types explicitly for PHP 8.4 compatibility

[W0rma]

https://wiki.php.net/rfc/deprecate-implicitly-nullable-types deprecates implicitly nullable parameter types in PHP 8.4

[TavoNiievez]

Hi @W0rma,

It seems that it is now possible to test with github actions PHP 8.4, see 8.4-rc, I am approving these PRs considering the change you apply and the reason why you do it, but in the future it would be appropriate to test that PHP 8.4 compatibility is something that is actually working.

Then, we should instead adapt the actions to verify that we have no deprecations, because technically we are already compatible with PHP 8.4, this change is strictly required for PHP 9.0.

Finally, feel free to tag me in the PRs you consider urgent, as long as I have ever used the repository I will be able to help you by reviewing them. Currently I only have notifications enabled for codeception, module-symfony and symfony-module-tests.

[W0rma]

@TavoNiievez Sorry, I should have used a better title for my MRs.

In fact they only fix a deprecation warning in PHP 8.4.

Seems like phpstan is not yet compatible with PHP 8.4 (see phpstan/phpstan#10182), therefore the PHP 8.4 would currently fail.