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

Change-Id: I0ef18509fecec56df9c922c44c81ffd5e02eae47
Reviewed-on: http://review.couchbase.org/40756
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>