Prepare for PHP 8.4

[staudenmeir]

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

The CS fixer rule nullable_type_declaration_for_default_null_value can also be removed, as true is its default value.

Tests

I tried adding 8.4 to the list of tested PHP versions in ci.yml, but the package requirements can't be resolved yet:

[mnapoli]

I liked the ?ObjectDefinition notation, are we able to configure the CS to use that?

Also I'm not sure why the build fails now 😕

[staudenmeir]

I liked the ?ObjectDefinition notation, are we able to configure the CS to use that?

How do you mean? What does that look like in comparison to ?ObjectDefinition $definition = null?

Also I'm not sure why the build fails now 😕

The CS fixer still fails because it no longer accepts return types like string|null and wants to replace them with ?string. There must have been a change in the default rules. This also affects other PRs like #886.

For this PR, I originally just ran php-cs-fixer fix and that also changed the return types. I reverted these changes in the second commit because that's a different topic, IMO. I can of course restore the return type changes to fix the CS build.

[mnapoli]

OK let's merge this for now 👍