Explicitly mark nullable values as nullable

[gjuric]

Fixes PHP 8.4 deprecation notices.

To view these notices before this commit, run tests with --display-deprecations flag on PHP 8.4.

[artursreiljans]

Sorry, just noticed that I've created a duplicate pull request (#95).

I think I'd leave mine open however, as it also intends to drop support for older PHP versions that are not compatible with modern syntax.

Anyway, it's kinda funny that we both created same pull requests few hours apart, even though PHP 8.4 was released almost two months ago :)

[gjuric]

Thank you, I've forgot to remove the unsupported PHP versions, so I will be closing this one.