Implicit nullable parameters are deprecated in PHP 8.4

Since PHP 8.4, implicit nullable parameter types are deprecated. Running WP OAuth Server on PHP 8.4 causes a slew of deprecation warnings such as:

[08-Apr-2026 13:57:32 UTC] PHP Deprecated:  WPOAuth2\Server::grantAccessToken(): Implicitly marking parameter $response as nullable is deprecated, the explicit nullable type must be used instead in .../plugins/oauth2-provider/library/WPOAuth2/Server.php on line 261

See: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

Function definitions should be altered to add the explicit nullable types using the ? character, like so:

public function grantAccessToken( RequestInterface $request, ResponseInterface $response = null ) {

# Should be changed to:

public function grantAccessToken( RequestInterface $request, ?ResponseInterface $response = null ) {

The PHP linter does not appear to find occurrences of this, but (thanks to the consistent formatting of this plugin’s code) the following command will reveal many of them:

grep -RIPs 'function.*[^,(] \$[a-zA-Z0-9]+ = *null' ../plugins/oauth2-provider/