Use hiddenname for hardestroom if roomname is empty

In earlier 2.3, if the roomname was empty, Dimension VVVVVV was used
instead. However, instead of doing that, it's better to just use the
hiddenname instead. Both because it's less hardcoded, and some rooms
have hidden names that aren't Dimension VVVVVV.

desktop_version/src/Game.cpp

@@ -3962,7 +3962,7 @@ void Game::gethardestroom(void)
         }
         else if (map.roomname == "")
         {
-            hardestroom = "Dimension VVVVVV";
+            hardestroom = map.hiddenname;
         }
     }
 }