[6.x] Fix PHP 8.4 deprecation warnings for implicit nullable types

[selfsimilar]

Resolve PHP 8.4 notices for deprecated 'Implicitly marking parameters as nullable'

Mostly annotating types as nullable by prepending a question mark.

Explanation

PHP 8.4 has a core deprecation of implicitly nullable parameter types. This PR resolves that issue.

Checklist

• I've read this template
• I've checked reviewed this PR myself, ensuring consistency and quality with the rest of the project
• I've given a good reason as to why this PR exposes new / removes existing user data

[joelbutcher]

Adding a note that, whilst accepting of this change, we don't officially support PHP 8.4 yet