MembershipManagerImpl.updateSubscription fails if already empty subscription

The internal SubscriptionState object keeps track of whether the assignment is user-assigned, or auto-assigned. If there are no assigned partitions, the assignment resets to NONE. If you call SubscriptionState.assignFromSubscribed in this state it fails.

The easiest thing is perhaps to check SubscriptionState.hasAutoAssignedPartitions() to make sure that assignFromSubscribed is going to be permitted.