When listening for InventoryClickEvent targetting virtual Anvil inventories, the result of getCurrentItem() is always null for the result slot.
This does not apply to normal Anvil inventories, only such opened by calling HumanEntity#openInventory():
```
        var anvil = Bukkit.createInventory(null, InventoryType.ANVIL, title);
        var label = new ItemStack(Material.NAME_TAG, 1);
        anvil.setItem(0, label);
        player.openInventory(anvil);
```

I have set up a minimal reproduction example project: https://github.com/burdoto/SPIGOT-7803