fix crash in compaction scheduler when there's no buckets

Change-Id: If0992905b1d082921b8e9eedea0d643873094a1a
Reviewed-on: http://review.couchbase.org/39371
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>