VerifyReplication may fail to count bad rows when regions of table in source cluster are empty

VerifyReplication will start the scanner for the table of peer cluster in Verifier#map(...). If the regions of the table in source cluster are empty, the Verifier#map(...) won't be invoked, so that the tool can not count the bad rows in the table of peer cluster. A possible improvement is to check whether the scanner for the table of peer cluster has been opened in Verifier#cleanup(...), and if not, open the scanner and count the rows only in the table of peer cluster as bad rows. Suggestions and discussions are welcomed.