I had a curious problem using the "null" tiles that is funny enough to report here.
The problem is Wayward workers and loose larva!
In some cases the drones mine but then go off the nul tiles. But even more bizare are the larva. In some cases they leave the creep and start wandering around the normal tiles! Totally unheard of before in Starcraft as far as I know.
In the first picture there is a larva wandering away from the hatchery "floating" in space! You can also see some stranded drones with minerals.
In the second picture you see 7 stranded larva foot loose and creep free!
Apparently the "null" terrain in the "space" maps has the star background. According to mapghost he uses tile numbers 1-15 when you select the tileset index menu. I still can't find any numbered tiles in my editor scmdraft2. The only numbers I see are the ones for the shape of the pointer. So I have just been copying and pasting the null terrain from his map "Mazicolozist".