Item modifiers that change the count to 0 don't completely remove the item

The bug

When subtracting from the count of an item with /item ... modify and it reaches 0, it isn't completely removed. This happens both on entities and containers.

How to reproduce

1. Add the data pack

1. to the world and /reload

2. Place any item in the first hotbar slot

3. /item entity @s hotbar.0 modify test:remove_count

4. /item entity @s hotbar.0 modify test:add_count

5. Notice that the original item is back. If you had relogged between step 3 and 4 this would have failed.