You can't test for an empty slot in the inventory.

Right now when you want to decect an empty inventory slot you have to testfor when an item isn't there anymore. If the id:"minecraft:air" or something similar existed you could cut down on A LOT of command blocks.

This fix to the Inventory tag would be very useful in multiple ways.
1. If you want a player not to throw away a specific item in their inventory.
2. If you want to create a shop in the player's inventory, you need to be able to detect if an inventory slot is empty.

Thanks for taking in consideration