Deprecated Implicit Null Parameters

[Zykaris]

Obviously this is pretty new, but with the general release of PHP 8.4.1 today, implicit null parameters now raise warnings. These warnings all say something similar to the following:

PHP Deprecated: Firebase\JWT\JWT::decode(): Implicitly marking parameter $headers as nullable is deprecated, the explicit nullable type must be used instead

This occurs in--at least--the following places:

1. src\JWK.php on line 55
2. src\JWK.php on line 96
3. src\JWT.php on line 96
4. src\JWT.php on line 203
5. src\JWT.php on line 204

Once Null is added to the type-hints, the library appears to work without issue.

[sbuerk]

Should be aready fixed with #572 which is merged. Sadly, still a release is missing #578

I'd say this can be closed as duplicate of #570

[Zykaris]

Duplicate of #570