make Tests\Fixtures\App\Model\DummyUser::eraseCredentials() compati…

…ble with `Symfony\Component\Security\Core\User\UserInterface::eraseCredentials()`

tests/Fixtures/app/Model/DummyUser.php

@@ -62,7 +62,7 @@ public function setUsername(string $username): void
         $this->username = $username;
     }
 
-    public function eraseCredentials()
+    public function eraseCredentials(): void
     {
         $this->password = null;
     }