CBQE-0: fix potential test code issue for merge_keys for bi-directional xdcr. Previously merge keys have None value in KvStore after merge and validation skipped.

Change-Id: I948128d622bf5bab3219d8071afbe0c42ec317ac
Reviewed-on: http://review.couchbase.org/40330
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>