I created a superflat world with just one layer of air using the superflat preset 3;air;1; and I can't move at all.I used 1.8 to create the world and I could move perfectly.In 15w34d,I can also see "holograms" of blocks at different areas which are non-existent.When I move towards these "holograms",the sky would darken,which happens when players are falling into the void.
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I ran into the same bug trying to create a superflat world with only air in snapshot 15w34d.
Presets I tried:
3;minecraft:air;1
3;0;1
Both create an air-only world, but you are unable to move outside of teleporting.
While in this immobile world, I used /setblock ~ ~ ~ minecraft:stone to create a block at my legs. It created the block but didn't push me out of it like in 1.8. I stayed standing within the block, unable to move.

Empty chunks are just entirely broken in this snapshot. They seem to behave almost identically to how chunk errors worked in 1.8.