fix handling of start rebalance from old node

Old code doesn't pass list of delta-recovery buckets, and in initial
code we handle it by converting it to false. But latest code doesn't
understand false as value of DeltaRecoveryBuckets. So we change it to
[] which has similar meaning to false in old code.

Change-Id: I352e75f1d42eda0655d5c2a66804a51f745743c4
Reviewed-on: http://review.couchbase.org/38444
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>