Crash: failed to serialize empty item stack in inventory

https://piston.java.mojang.zone/java/versions/24w18a/crash/6266382/15900506

Not sure how this happens - in the worst case, it's a threading issue (we check that the stack is not empty before serialising)
On the other hand, it seems more likely to be an empty item stack inside of the top-level item stack (e.g. inside of a bundle/container). Needs investigation to figure out how that state is created, and probably benefits from validation at the point of creation.