public static <U, V> UpdateMessage<U, V> of(U update) {
    return new UpdateMessage<>(update, null);
  }
Represents an update and an optional default record to be inserted for a key,
if the update is applied to a non-existent record.

@param <U> type of the update record
@param <V> type of the default record
samza-api/src/main/java/org/apache/samza/operators/UpdateMessage.java