ISPN-4673 Split-brain: get() returns null when all owners are removed from view

https://issues.jboss.org/browse/ISPN-4673

* If get returns null, check that at least one owner is in the cluster.
* Replace RpcException with AvailabilityException if there are no valid
  answers.
No, the check there uses `containsAll` and only applies if the local node is already in degraded/unavailable mode.