@Test void parentIdFromLong_zeroSameAsNull() {
    assertThat(base.toBuilder().parentId(0L).build().parentId())
      .isNull();
    assertThat(base.toBuilder().parentId("0").build().parentId())
      .isNull();
  }
Some tools like rsocket redundantly pass high bits as zero.
zipkin/src/test/java/zipkin2/SpanTest.java