MAPG-199 fix issues found by PHPStan

src/Controller/MapAdminController.php

@@ -161,7 +161,7 @@ class MapAdminController implements ISecured
         return new JsonContent(['mapId' => $map->getId(), 'added' => $addedIds]);
     }
 
-    public function deleteMap()
+    public function deleteMap(): IContent
     {
         $mapId = (int) $this->request->query('mapId');