Fix unnecessary exception use in Map::isNodeUnderground

Fix unnecessary exception use in Map::isNodeUnderground

The isNodeUnderground calls getBlockNoCreate which calls
getBlockNoCreateNoEx and throws InvalidPositionException
if the returned value is nullptr, which isNodeUnderground
then catches to return "false". Remove the try..catch in
isNodeUnderground by calling getBlockNoCreateNoEx instead
of getBlockNoCreate and checking the returned value for
nullptr.