MB-11970: Resident ratio should be 100% when no items are a bucket

Change-Id: Ia0cf40865d377b97e50c3eca82a55b056d044280
Reviewed-on: http://review.couchbase.org/40759
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>