Ender dragon respawns when upgrading from 1.12 to 1.13-pre3

The bug

When upgrading from 1.12.2 to 1.13-pre3, the ender dragon respawns.

Steps to reproduce

1. Create a world in 1.12.2
2. Go to the end and kill the ender dragon
3. Leave the end and return to the overworld
4. Exit the game and join the world in 1.13-pre3
5. Go to the end
   → Notice the ender dragon has respawned