I've generated a superflat world with the "The Void" preset (biome: minecraft:the_void) to do some Creative things. I believe that mob spawning is disabled for this biome; is there any way to enable mob spawning? I've tried /gamerule doMobSpawning true, with no success.
You can edit the preset to change the biome to another biome, such as plains, which will replicate the "void" preset except there won't be a stone platform (you can use /setblock to create one or place an initial block so you can then place more normally):
As for changing the biome for an existing world, specifically, already generated chunks, there is no easy way to do that in 1.13 (you could use MCEdit to change the biome but it does not support the new save format yet. New chunks can be changed by using NBTExplorer, which does support 1.13, to edit the "generatorOptions" string to change the biome).