Fill old terrain below 0 with bedrock

When a player migrates their old worlds to new versions (1.17 snapshot), the world generation is now below 0.

At the boundary of old chunks and newly generated chunks, the player can find themselves staring into the void from below bedrock of previous chunks.

1) This is the simplest way to avoid this is to fill terrain below 0 with bedrock in old chunks.

2) The alternative is to leave the world as-is without modifying chunks.

Either solution is fine. It is just a matter of preference.

The first solution is probably more immersive, and could make for some really cool bedrock bases for advanced players, lol. The second solution is probably more nostalgic.