When creating a floating islands world without selecting a biome, the "Ocean" biome is selected by default which causes the world to be almost completely empty (see MC-190083).

To reproduce

1. Create a Floating Islands world using default options
2. Enter world
3. You most likely fall out of world and die
4. Re-create in creative with cheats, and see there are no islands around you--probably not
5. Repeat until you spawn on an island
6. Try with seed 2095025955442124028. You spawn on small island at somewhere around -29.5 49 111.5 (seems to be different each time, but on that island). There is one very small island in sight

So, if no islands within a certain distance, you spawn at 8.5 64 8.5

If there is an island close to that, in which case you will spawn there. I tried ten times and only landed on the one island I mention.