static MessageListener create(MessageListener delegate, JmsTracing jmsTracing) {
    if (delegate == null) return null;
    if (delegate instanceof TracingMessageListener) return delegate;
    return new TracingMessageListener(delegate, jmsTracing, true);
  }
@Test void null_listener_if_delegate_is_null() {
    assertThat(TracingMessageListener.create(null, jmsTracing))
      .isNull();
  }