return empty array if no servers, buckets, metrics, events found

Change-Id: I8a0da1b885ef048e84f232e8a268dcef4b14deaa
Reviewed-on: http://review.couchbase.org/23777
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>