MAPG-199 fix issues found by PHPStan

src/Controller/GameController.php

@@ -32,7 +32,7 @@ class GameController
         return new JsonContent($this->prepareGame($mapId));
     }
 
-    private function prepareGame(int $mapId)
+    private function prepareGame(int $mapId): array
     {
         $map = $this->mapRepository->getById($mapId);