MB-8815 Fix RO user access to auth_any_bucket endpoints.

This didn't work if there were no buckets defined.

Change-Id: I0693715c3a92d3f97e8c135265fdb056796236fc
Reviewed-on: http://review.couchbase.org/28605
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>