EC: Verify unrecoverable EC containers which are empty transition to deleting

In EC, a container is considered "missing" if there are not enough replicas to reconstruct it. If a key is deleted from one of these containers, the blocks should still be deleted from the replicas that are present. When all the blocks are deleted, the remaining replicas will be empty so the container should be deleted. It looks like this already happens in the code, so this Jira is to add a unit test for this case as well.