Review consumer onPartitionsAssigned called with empty partitions

Legacy coordinator triggers onPartitionsAssigned with empty assignment (which is not the case when triggering onPartitionsRevoked or Lost). This is the behaviour of the legacy coordinator, and the new consumer implementation maintains the same principle. We should review this to fully understand if it is really needed to call onPartitionsAssigned with empty assignment (or if it should behave consistently with the onRevoke/Lost).

Note that the consumer integration tests rely on this call to onPartitionsAssigned to #awaitRebalance (AbstractConsumerTest.scala)