fix: prevent empty requests from paused consumer

When using the new "pause consumer" support (IBM#2005), Sarama would
incorrectly submit empty FetchRequests if all of the assigned partitions
were paused. This is because we only used Pause to skip adding the
topicPartition blocks to the FetchRequest and still went ahead and sent
the Fetch even if it was essentially empty:

consumer.go

@@ -936,6 +936,12 @@ func (bc *brokerConsumer) subscriptionConsumer() {
 			return
 		}
 
+		// if there isn't response, it means that not fetch was made
+		// so we don't need to handle any response
+		if response == nil {
+			continue
+		}
+
 		bc.acks.Add(len(bc.subscriptions))
 		for child := range bc.subscriptions {
 			child.feeder <- response
@@ -1036,6 +1042,8 @@ func (bc *brokerConsumer) abort(err error) {
 	}
 }
 
+// fetchResponse can be nil if no fetch is made, it can occur when
+// all partitions are paused
 func (bc *brokerConsumer) fetchNewMessages() (*FetchResponse, error) {
 	request := &FetchRequest{
 		MinBytes:    bc.consumer.conf.Consumer.Fetch.Min,
@@ -1077,5 +1085,10 @@ func (bc *brokerConsumer) fetchNewMessages() (*FetchResponse, error) {
 		}
 	}
 
+	// avoid to fetch when there is no block
+	if len(request.blocks) == 0 {
+		return nil, nil
+	}
+
 	return bc.broker.Fetch(request)
 }