Optional<JsonNode> missingValueFor(String fieldName) {
    if (!fields().containsKey(fieldName)) {
      final String message = String.format(Locale.ROOT,
              "Field %s not defined", fieldName);
      throw new IllegalArgumentException(message);
    }

    return fields().get(fieldName).missingValue();
  }
Used as special aggregation key for missing values (documents that are
missing a field).

<p>Buckets with that value are then converted to {@code null}s in flat
tabular format.

@see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-sum-aggregation.html">Missing Value</a>
criteria/elasticsearch/src/org/immutables/criteria/elasticsearch/Mapping.java