Add explicit nullable types to fix implicit-nullable deprecations on PHP 8.4

[m-ober]

Straight forward fix to avoid deprecation messages on newer PHP versions.

[parallels999]

Unfortunately, @mike42 has abandoned this package.
Your changes are very necessary

[mike42]

Much appreciated @m-ober - this package is long overdue for a maintenance release just to adjust PHP version support, and this helps flag a specific improvement to ship with that.

I unfortunately need to fix up build/test infra before I can ship this, watch this space.

[m-ober]

Hey, thanks for the response!

If you need help with that, I could support you by writing some GitHub action files for build/test.