[Garkin]

I think that the easiest solution is checking if arguments are valid before you use them.

--for our purposes are valid bags:
-- BAG_WORN (0)
-- BAG_BACKPACK (1)
-- BAG_BANK (2)
-- BAG_GUILDBANK(3)
--Other bags are invalid:
-- BAG_BUYBACK (4),
-- BAG_TRANSFER (5)
-- BAG_DELETE(255)
 
local function IsSlotOccupied(bagId, slotIndex)
    if (type(bagId) == "number" and bagId >= 0 and bagId <= 3) then
        if (type(slotIndex) == "number" and slotIndex >= 0 and slotIndex <= GetBagSize(bagId) - 1) then
            local _, stackCount = GetItemInfo(bagId, slotIndex)
            return stackCount > 0
        end
    end
end
--function returns:
--true: bag slot is occupied
--false: bag slot is empty
--nil: invalid bagId or slotIndex arguments

This is how it does inventory manager:

function ZO_InventoryManager:IsSlotOccupied(bagId, slotIndex)
    local inventoryType = self.bagToInventoryType[bagId]
    local slot = self.inventories[inventoryType].slots[slotIndex]
 
    return ((slot ~= nil) and (slot.stackCount > 0))
end

Global reference to that function is PLAYER_INVENTORY:IsSlotOccupied(bagId, slotIndex)