Strongholds generate floating and without walls, floors, or ceilings when inside caves

The bug

Because world generation has changed in 21w06a, large portions of strongholds can generate without floors. This is much more of an issue in 21w06a than it was in MC-70705, so this clones that.

How to reproduce

1. Generate a world with seed 7624679963404965487
2. Run the command:

   /execute in minecraft:overworld run tp @s 8181.02 12.53 -1423.15 -209.75 -14.10 

   Observe the floating stronghold libraries