I want to make a ghost walk on space, I know you can do it, and i think it's something about null terrain, but I don't exactly know how it works. How would you do that?
None.

NudeRaider -- blessed innocent soul knows nothing of the strife we had before EUDs were discovered 
DarkenedFantasies -- Probably just didn't care. For example, at some point before release, they've updated the graphics of some of the Protoss buildings (Forge, CyberCore, Citadel, Observatory, Arbiter Tribunal), but instead of properly re-rendering them with edited 3D models, they did crappy copy-paste jobs on the rendered graphics.