How to check if a slot is empty?

I have this mcf replace entity @a[nbt={Inventory:[{id:"minecraft:air",Slot:8b}]}] hotbar.8 with compass[lodestone_tracker={target:{dimension:"minecraft:overworld",pos:[I;$(varX),$(varY),$(varZ)]},tracked:true},custom_name={"bold":true,"color":"dark_green","text":"Rune of the hunt"}] 1 How do i check if the slot on 8b is empty?