Situation & Action:
The player has opened a double chest and his own inventory is full (there is no empty space left). He then moves his cursor over an item in the chest part of the inventory and presses a hotkey (1-9) on his keyboard. The two items will switch and the player connection fires the InventoryClickEvent.

Problem:
In this special case, we land on this line and the InventoryAction is set to NOTHING.

This poses a problem with our protection plugin, we evaluate whether the player is giving or taking from/to and inventory according to the InventoryAction and check if he is allowed to perform this action. Swaps are usually both: giving and taking.
In a case where the chest part is protected, one should not be allowed to swap these two items if they don't own the chest.

Suggested Solution:
In my opinion, this is definitely not "nothing", but rather an actual "hotbar swap". The behaviour of this situation is exactly what the javadocs of "HOTBAR_SWAP" sais:
"The clicked slot and the picked hotbar slot are swapped."