Custom GUIs' item dragging produces one too many per slot

[Mikedeejay2]

This bug is from #59 where for some reason where in custom GUIs when dragging items in the GUI one too many items is created per slot. The reason for this is because in normal GUIs, an empty item is simply AIR x 0, where an empty item in a custom GUI for whatever reason is null, since the algorithm can't get the amount of items in null, it skips that item and leaves the previous 1 item amount in that slot. This is what causes the bug.