static @Nullable String emptyToNull(Record next, TableField<Record, String> field) {
    String result = next.getValue(field);
    return result != null && !result.isEmpty() ? result : null;
  }
Assumes the input records are sorted by trace id, span id
zipkin-storage/mysql-v1/src/main/java/zipkin2/storage/mysql/v1/DependencyLinkV2SpanIterator.java