@Test void span_nullServiceName() {
    String json =
      """
      {
        "traceId": "6b221d5bc9e6496c",
        "name": "get-traces",
        "id": "6b221d5bc9e6496c",
        "localEndpoint": {
          "serviceName": null,
          "port": 65535
        }
      }
      """;

    assertThat(parse(SPAN_PARSER, json).localEndpoint())
      .isEqualTo(Endpoint.newBuilder().serviceName("").port(65535).build());
  }
This isn't a test of what we "should" accept as a span, rather that characters that trip-up
json don't fail in SPAN_PARSER.
zipkin-storage/elasticsearch/src/test/java/zipkin2/elasticsearch/JsonSerializersTest.java