[Blocks] Air exists in item form

[BlueStaggo]

For OSL 0.20.0-alpha.1

In 1.6.4, items with an ID of 0 do not get removed. Instead, they appear as an items with a missing texture and the name "tile.null.name":

As such an item does not exist in any version of Minecraft outside of 13w38b, air items should not exist in OSL either.