There's no easy way to get a variable card that contains an air block, since using "Block" in a logic programmer with nothing in it says "The inserted item is no block." and refuses to write the card. (The hard way is to use a materializer on a block reader's output or the "blockbyname" operator on "minecraft:air".) "Item" currently will write an empty item stack if you try, so I think "Block" should work the same way.