Some brushes ignore cave air

[HauserGrim]

Some brushes cannot change cave air, I think, in addition to AIR material, it is worth checking CAVE_AIR and VOID_AIR. https://github.com/EngineHub/WorldEdit/blob/master/worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/BukkitBlockRegistry.java#L85