I'm fairly confident that nebulas seed even if there are no champions at gamestart. I believe I've done this in the past, and you'll even notice the random tachyon guardians in the relevant location. I'll check and be back in a few minutes.
Yep, as soon as I add a second (disabled) champion player or switch myself to normal+champion, the nebulas appear. I believe this is because they exist even if you don't see them. There even used to be mysterious wormhole guard posts in the middle of nowhere.