@Nullable static String lastStringHeader(Headers headers, String key) {
    Header header = headers.lastHeader(key);
    if (header == null || header.value() == null) return null;
    return new String(header.value(), UTF_8);
  }
@Test void lastStringHeader_null() {
    assertThat(KafkaHeaders.lastStringHeader(record.headers(), "b3")).isNull();
  }