What and why? 🤔

Let's start testing the library on PHP 8.4 and fix issues. Specifically, implicit nullable are now deprecated as of PHP 8.4:

- public function __construct(CacheProvider $cache_provider = null)
+ public function __construct(?CacheProvider $cache_provider = null)

Unless we make the change above, we'll see:

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

This is a non-breaking change, as you can see from the lack of LSP errors: https://3v4l.org/VmME0

Testing Steps ✍️

CI should pass, review the changes.

You're it! 👑

Tag a random reviewer by adding @Automattic/stream-builders to the reviewers section, but mention them here for visibility as well.