Fix: Remove Implicitly nullable parameter declarations deprecations (#56)

Author: eneko-koraliving

src/Action/RegisterSubscriptionAction.php

@@ -47,7 +47,7 @@ private function unsubscribe(UserInterface $user, string $subscriptionHash): voi
         $manager->delete($subscription);
     }
 
-    public function __invoke(Request $request, UserInterface $user = null): Response
+    public function __invoke(Request $request, ?UserInterface $user = null): Response
     {
         if (null === $user) {
             throw new AccessDeniedHttpException('Not authenticated.');

src/Sender/PushMessageSender.php

@@ -48,7 +48,7 @@ class PushMessageSender implements PushMessagerSenderInterface
     public function __construct(
         array $auth = [],
         array $defaultOptions = [],
-        ClientInterface $client = null
+        ?ClientInterface $client = null
     ) {
         if (isset($auth['VAPID'])) {
             $auth['VAPID']['validated'] = false;