Sugar is Zone 0’s Gaurdian. The reason why no one else is in Zone 0 is because she ate them all (the Elsen are made of sugar, and sugar is addicting…). When word reached the Queen, the Queen had her locked up where surely no one would find her. This is the reason why Zacharie said “It’s better this way,” after Batter kills her. She would surely not be able to eat any others if she were dead.
The Toad King was the Guardian of Zone 0. He was corrupted and mean-spirited though, and treated his ‘subjects’, the inhabitants, badly. In the end, the was slain by Zacharie for it, with some help from Sugar/Sucre. Zacharie then made the Toad mask out of the Toad King’s remains, and Sugar/Sucre became the new guardian of Zone 0.
Zone 0 never had a guardian and was always independent from the Queen’s kingdom. That’s why it cannot be purified and why Elsen don’t live there. They are too afraid to live where there is no guardian to protect them.
My headcannon for the nothingness is, that there were more zones. When you go to the nothingness, you can find spots on the map that are large enough for there to be more zones, yet when you walk over to it; nothing happens. Perhaps the batter destroyed it before the game had started. Perhaps the queen had gotten rid of it. Who knows.