CBIDXT-133: workaround to send nil secondary-keys.

Change-Id: I6bb8aed8d4c92b10ee98d48565d2d87b9f9d04ae
Reviewed-on: http://review.couchbase.org/40822
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>