[5.2] [PHP8.4] Update for nullable type declaration

[heelc29]

Summary of Changes

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

Testing Instructions

at the moment no QA release of PHP8.4 ... so only code review (and phpcs by drone)

Actual result BEFORE applying this Pull Request

Warning: Deprecated: Implicitly marking parameter $var as nullable is deprecated, the explicit nullable type must be used instead

Expected result AFTER applying this Pull Request

No new deprecated warnings if update to PHP8.4 is available

Link to documentations

Please select:

• [x] No documentation changes for docs.joomla.org needed
• [x] No documentation changes for manual.joomla.org needed

[HLeithner]

This pull request has been automatically rebased to 5.2-dev.

[laoneo]

Thanks!