Run CI tests on PHP 8.4 and fix deprecations

[jnoordsij]

WHY

BEFORE - What was wrong? What was happening before this PR?

Tests were not run on the latest PHP version (8.4; see https://www.php.net/releases/8.4/en.php).

AFTER - What is happening after this PR?

Tests are run and less deprecations are triggered.

HOW

How did you achieve that, in technical terms?

• Fix implicit nullable parameter declarations; see also https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

Is it a breaking change?

No

How can we test the before & after?

If the PR has changes in multiple repos please provide the command to checkout all branches, eg.:

git checkout "dev-branch-name" &&
cd vendor/backpack/crud && git checkout crud-branch-name &&
cd ../pro && git checkout pro-branch-name &&
cd ../../..

[pxpm]

Thanks, @jnoordsij

I've tested this, and while doing so I also found a failing test in php 8.4.

I will merge this, and the fix after 👍

Cheers