Old worlds with non-bedrock blocks at 0,0,0 do not upgrade chunk column from 0,0,0 to 0,-64,0

The Bug

Old worlds with non-bedrock blocks at 0,0,0 when upgraded do not convert the ground under them into deepslate

Reproduce

• Open a world in 1.11.2
• Go down to 0,0,0
• Replace the bedrock with any other block
• Leave and open the world in 21w43a or above

Observed Result

A column was made that went to the void

Expected Result

There would be deepslate in the column not a gap