It is possible for a container to be brought into the world pre-filled
in place event (thanks NBT!). When the blockplacevent(s) are cancelled
their blockstate is updated in OBC. This triggers them to drop their
inventories.
Rather than screwing with the update method which is used elsewhere, we
can simply clear any inventories once we are told that a plugin intends
to cancel the interaction.
Fixes GH-1831
