Evaluate nimbus.groups when nimbus.users is empty

[rzo1]

SimpleACLAuthorizer.permit() short-circuited on nimbusUsers.size() == 0 and allowed the call, so nimbus.groups was never consulted when nimbus.users was left empty. An operator locking the cluster down by group alone therefore got no restriction at all.

All four combinations are now evaluated; both-empty still means no restriction, which is the shipped default. Extends SimpleACLAuthorizerTest.

Note: a cluster that set nimbus.groups without nimbus.users will now restrict user-level commands to members of those groups. That includes NimbusClient.getConfiguredClientAs, which calls getLeader() on every connection.