Empty modified chunk does not get saved

When emptying (clearing all blocks) in a chunk, that chunk doesn't get saved upon reload.

To reproduce

1. Create a new flat world

2. Look around (This is to ensure all of the chunks loaded around you)

3. Dig one block down

4. Run the following command:

   /fill ~20 ~ ~20 ~-20 ~-3 ~-20 air

5. Save, exit, then open the world

Expected result

• The blocks between -20 -64 -20 to -20 -60 -20 should be empty upon reload

Observed

• Blocks between 0 -64 0 to 15 -60 15 doesn't get cleared upon reload