@Test
public void mapFromMessageWithoutPayloadWithoutContentType() {

    final DittoHeaders dittoHeaders = DittoHeaders.newBuilder().randomCorrelationId().build();
    final Message<Object> message = messageBuilder(null).build();
    final Signal<?> sendThingMessage = SendThingMessage.of(THING_ID, message, dittoHeaders);
    final List<ExternalMessage> result = underTest.map(ADAPTER.toAdaptable(sendThingMessage));
    assertThat(result).hasSize(1);
    final ExternalMessage externalMessage = result.get(0);
    assertThat(externalMessage.getBytePayload()).isEmpty();
    assertThat(externalMessage.getTextPayload()).isEmpty();
    assertThat(externalMessage.getHeaders()).containsAllEntriesOf(message.getHeaders());
}