PubsubMessageWithAttributesCoder should not produce null attributes map

public class PubsubMessageWithAttributesCoder extends CustomCoder<PubsubMessage> {
  // A message's payload can not be null
  private static final Coder<byte[]> PAYLOAD_CODER = ByteArrayCoder.of();
  // A message's attributes can be null.
  private static final Coder<Map<String, String>> ATTRIBUTES_CODER =
      NullableCoder.of(MapCoder.of(StringUtf8Coder.of(), StringUtf8Coder.of()));

  @Override
  public PubsubMessage decode(InputStream inStream) throws IOException {
    return decode(inStream, Context.NESTED);
  }

  @Override
  public PubsubMessage decode(InputStream inStream, Context context) throws IOException {
    byte[] payload = PAYLOAD_CODER.decode(inStream);
    Map<String, String> attributes = ATTRIBUTES_CODER.decode(inStream, context);
    return new PubsubMessage(payload, attributes);
  }
}

public class PubsubMessage {

  private byte[] message;
  private Map<String, String> attributes;

  public PubsubMessage(byte[] payload, Map<String, String> attributes) {
    this.message = payload;
    this.attributes = attributes;
  }

  /** Returns the main PubSub message. */
  public byte[] getPayload() {
    return message;
  }

  /** Returns the given attribute value. If not such attribute exists, returns null. */
  @Nullable
  public String getAttribute(String attribute) {
    checkNotNull(attribute, "attribute");
    return attributes.get(attribute);
  }

  /** Returns the full map of attributes. This is an unmodifiable map. */
  public Map<String, String> getAttributeMap() {
    return attributes;
  }
}