OffsetSyncStore.translateDownstream() should be compatible with the scenario where no msgs are sync-ing while consumer group offsets are sync-ing

Let's say, no msgs in source topic parititon (A-0) because of short ttl ,but there are still 
offsets committed by consumers subscribing A-0. At this time ,we set up mm2, and it begins to 
sync A-0 and the according consumer group ,too. Then we will find it hard to understand that:
even though there are no msgs beging synced for real , consumer group offsets are still beging
synced to target cluster ,which would producing lag <0 in target cluster for A-0.