Sample has an incorrect call to `consumer.accept()` in lines [37](https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/MessageReceiver.java#L37) and [39](https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/MessageReceiver.java#L39). The `null` parameter shouldn't be there.