Nullable method parameters require "?" in PHP 8.4

[shu-yusa]

Description

From PHP8.4, a parameter that has null as a default value has to be typed as a nullable type: https://www.php.net/manual/en/migration84.deprecated.php
There are several places where method parameters take null as a default value but ? is missing.

Steps to reproduce

Instantiate RSAKey object for example:

bash-3.2$ php -a
Interactive shell

php > require 'vendor/autoload.php';
php > $key = new \SimpleJWT\Keys\RSAKey("{}", 'json');
PHP Deprecated:  SimpleJWT\Keys\Key::toJWK(): Implicitly marking parameter $password as nullable is deprecated, the explicit nullable type must be used instead in /***/simplejwt/src/SimpleJWT/Keys/Key.php on line 237

Deprecated: SimpleJWT\Keys\Key::toJWK(): Implicitly marking parameter $password as nullable is deprecated, the explicit nullable type must be used instead in /***/simplejwt/src/SimpleJWT/Keys/Key.php on line 237
php > 

Expected behaviour

Method is called without deprecation warning.

Environment

• Version: 1.0.2
• Server OS: Mac OS, Linux
• PHP: 8.4
• Packages: The same one as the master branch composer.json

Backtrace

Insert backtrace where relevant

Additional information

Any other information which you think would be useful in diagnosing the problem.