MB-100: minor fix in get_bucket_stats_for_node

when any stats with empty list

Change-Id: Id5b32c48c22c878a8cd7df9674400e215df874f5
Reviewed-on: http://review.couchbase.org/25191
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>