Cave world generation doesn't extend below y=0

The bug

In worlds that use the "Caves" world type, the lower bedrock layer is generated at y=0, rather than at y=-64. Blocks can still be placed below y=0, however nothing is generated there.

How to reproduce

1. Create a new world with "World Type" set to "Caves"
2. Travel below y=0:

   /tp ~ -2 ~

   → Observe that nothing is generated below y=0