code: @Nullable
      public Object fetchAValue() {
          if (doubleValue != null) {
              return doubleValue;
          }
          if (doubleArray != null) {
              return doubleArray;
          }
          if (longValue != null) {
              return longValue;
          }
          if (longArray != null) {
              return longArray;
          }
          if (stringValue != null) {
              return stringValue;
          }
          if (stringArray != null) {
              return stringArray;
          }
          if (map != null) {
              return map;
          }
          return null;
      }
docs: @return "the" value of this Payload, stuffed into an Object.
  This is used for evaluating the "validator" portion of a
  PayloadSpecification against these Payloads.
  
  We don't want the JsonSerializer to know about this, so
  renamed to not begin with "get".