TilemapParser.INSERT_NULL

When scanning the Tiled map data the TilemapParser can either insert a null value (true) or
a Phaser.Tile instance with an index of -1 (false, the default). Depending on your game type
depends how this should be configured. If you've a large sparsely populated map and the tile
data doesn't need to change then setting this value to true will help with memory consumption.
However if your map is small, or you need to update the tiles (perhaps the map dynamically changes
during the game) then leave the default value set.