PHP 8.4 support

[Jubeki]

As of PHP 8.4 parameters marked as implicit nullable will throw a deprecation warning. Explicitly Declaring is Supported as of PHP 7.1 (See https://www.php.net/manual/en/migration71.new-features.php). This package requires PHP >=7.2.5 so no lower bounds need to be updated.

This PR adds the question mark or null to the parameter types to to avoid the deprecation warning in PHP 8.4.

This was done with the help of laravel/pint and the following configuration in the pint.json:

{
    "preset": "empty",
    "rules": {
        "nullable_type_declaration_for_default_null_value": true
    }
}

[Jubeki]

Thanks!