pubsub/kafkapub: nil pointer dereference

`ExampleOpenTopic` (pubsub/kafkapubsub/example_test.go) show a
simple call to `OpenTopic` without `kafkapubsub.TopicOptions` as
passing a `nil` argument.
This is done by replacing a `nil` pointer with the appropriate
zero-value.  This behaviour exists in `OpenSubscription` too.
However, PR google#3163 introduced overriding batching options which
inadvertently accesses a field of a nil pointer.

pubsub/kafkapubsub/kafka.go

@@ -234,7 +234,11 @@ func OpenTopic(brokers []string, config *sarama.Config, topicName string, opts *
 	if err != nil {
 		return nil, err
 	}
-	bo := sendBatcherOpts.NewMergedOptions(&opts.BatcherOptions)
+	bo := &batcher.Options{}
+	if opts != nil {
+		bo = &opts.BatcherOptions
+	}
+	bo = sendBatcherOpts.NewMergedOptions(bo)
 	return pubsub.NewTopic(dt, bo), nil
 }