[php] Fix deprecation warning when running integration tests

OpenAPI\Client\FakeHttpClient::setResponse(): Implicitly marking parameter $response as nullable is deprecated, the explicit nullable type must be used instead

Author: simonhammes

samples/client/petstore/php/OpenAPIClient-php/tests/FakeHttpClient.php

@@ -27,7 +27,7 @@ public function getLastRequest()
     /**
      * @param null|ResponseInterface $response
      */
-    public function setResponse(ResponseInterface $response = null)
+    public function setResponse(?ResponseInterface $response = null)
     {
         $this->response = $response;
     }