This patch changes an internal assert (which can only be triggered
when a node group is empty) into properly handling this case (and
returning empty node/instance lists).

While we could handle this in the backend (Cluster.splitNodeGroup)
this would actually mean than we change the behaviour for a cluster
with just two node groups, once of which is empty (where today we
don't require a node group argument).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>