Implement serialization for inventory-keyless items

[FromDarkHell]

Certain items (i.e. ECHO Themes) can entirely lack an inventory key when we serialize the item; As long as it lacks actual parts, we don't really need the inventory key.
For now, the approach I've taken is to let the user be able to see but not modify the item if it lacks an inventory key, but in reality it only needs to block the addition of parts.