Only add dirty subscriptions to the request

Also handle if the request ends up empty because all of the subscriptions were
clean.

offset_manager.go

@@ -384,6 +384,10 @@ func (bom *brokerOffsetManager) mainLoop() {
 
 func (bom *brokerOffsetManager) flushToBroker() {
 	request := bom.constructRequest()
+	if request == nil {
+		return
+	}
+
 	response, err := bom.broker.CommitOffset(request)
 
 	if err != nil {
@@ -428,12 +432,20 @@ func (bom *brokerOffsetManager) constructRequest() *OffsetCommitRequest {
 		Version:       1,
 		ConsumerGroup: bom.parent.group,
 	}
+
 	for s := range bom.subscriptions {
 		s.lock.Lock()
-		r.AddBlock(s.topic, s.partition, s.offset, 0, s.metadata)
+		if s.dirty {
+			r.AddBlock(s.topic, s.partition, s.offset, 0, s.metadata)
+		}
 		s.lock.Unlock()
 	}
-	return r
+
+	if len(r.blocks) > 0 {
+		return r
+	}
+
+	return nil
 }
 
 func (bom *brokerOffsetManager) abort(err error) {