Bug: Session.Items.AllItemsReceived occasionally appears to append a null item

[alwaysintreble]

Occasionally AllItemsReceived has a null item appended to it, preventing clients that attempt to update their received items by reading this collection from getting the actual item, while they update their internal index. Unknown what exactly causes the issue, only know that manually tracking received items in the game client and occasionally verifying it against the lib collection would trigger them to be mismatched, despite the index being the same.