Testing for an empty slot does not work.

Yes, I have seen some reports saying that it is a feature suggestion, not a bug.

On the contrary, if you're telling it to find air (when there is air/nothing in the slot), it should come out as true, however it's saying that is not the case, as if there is something in that slot.

In a similar case, if you have the same item in the first 2 slots and test for {Items:[

{Slot:0b,Count:1b,Damage:0s,id:"minecraft:stone"}

,{}]} refering to the second slot being empty, it still shows as true, even though the second slot has data in it.