private static boolean isMissingBucket(JsonNode key) {
      return Mapping.Datatype.isMissingValue(key);
    }
Determines if current key is a missing field key. Missing key is returned when document
does not have pivoting attribute (example {@code GROUP BY _MAP['a.b.missing']}). It helps
grouping documents which don't have a field. In relational algebra this
would normally be {@code null}.

<p>Please note that missing value is different for each type.

@param key current {@code key} (usually string) as returned by ES
@return {@code true} if this value
criteria/elasticsearch/src/org/immutables/criteria/elasticsearch/Json.java