IsEmpty() method receiver for protobuf.TsVbuuid.

TopicResponse:rollbackTimestamps slice shall contain items only for buckets
will rollback.

Change-Id: I662109f8375825fe7ed991c8756200747a664170
Reviewed-on: http://review.couchbase.org/42460
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>