In old worlds, old chunks below 0 are empty

Summary:

In old worlds, old chunks below 0 are empty. According to the bedrock version, caves should be generated under the bedrock and the bedrock should be replaced with deepslate. But in Java, at the junction of old and new chunks, you can simply fall into the void.

Steps to reproduce:

1. Open the world created in 1.17 and below.

2. Visit the junction of old and new chunks below height 0.

Observed results:

You can easily fall into the void at the junction of new and old chunks.

Expected results:

According to the bedrock version, the bedrock at 0 height will be replaced by deepslate, and below 0 (in old chunks) there will be new caves.

However, earlier in the bedrock version, the old chunks below 0 were completely filled with the bedrock.

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