Bedrock layer does not generate at the bottom of the world, when upgrading world from 1.17.1 to 21w44a, in existing chunks.

Steps to Reproduce - 

1. Make a World in 1.17.1 and open it.
2. Open the World in 21w44a
3. Notice that there will be no bedrock at the bottom of the world, in chunks that were already generated in 1.17.1.