That is because you're assigning an empty tile (i.e. a tile with no data in it) as opposed to "no tile".
You can assign no tile in code by assigning spriteId of -1, or in the tilemap editor, clear a tile by command+click on mac, ctrl+click on pc.
In that case there will be no tile created.
There will be an optimization for physically empty tiles like yours in a later version, but that is rather low priority as there is a usable alternative to it right now.