[TOOLS] Add explicit return types to fix deprecation warnings raised by PHPUnit

plugins/OAuth2/Repository/Client.php

@@ -35,6 +35,7 @@ namespace Plugin\OAuth2\Repository;
 
 use App\Core\DB\DB;
 use App\Util\Exception\NotFoundException;
+use League\OAuth2\Server\Entities\ClientEntityInterface;
 use League\OAuth2\Server\Repositories\ClientRepositoryInterface;
 use Plugin\OAuth2\Entity;
 
@@ -63,7 +64,7 @@ class Client implements ClientRepositoryInterface
      * @throws \App\Util\Exception\DuplicateFoundException
      * @throws NotFoundException
      */
-    public function getClientEntity($clientIdentifier)
+    public function getClientEntity($clientIdentifier): ?ClientEntityInterface
     {
         return DB::findOneBy(Entity\Client::class, ['id' => $clientIdentifier]);
     }