Fallback to gm_flatgrass if the last played map is not found

If the last played map is not found then the map choice screen appears blank. This commit fixes this issue by falling back to gm_flatgrass in these cases.

The last played map may not be found if:
* the player has yet to load a map through the map choice screen after the update which started saving last maps (this includes newcomers)
* the map has been deleted since
* the cookies get cleared or corrupted in some way