Hey there, I'd recommend going into the Singleplayer "Flat" world options, and creating a world with the "void" preset.
Once this has been done and generated, copy the world folder onto your server and it should still be a void world.
Occasionally, the generator might not be copied, so it could be worth loading a few void chunks in singleplayer first, then copying the files with the preloaded void chunks first. You can set a smaller worldborder to stop players being able to create new chunks on the server to prevent terrain spawning.
I hope this helps.
