Validate FlinkReduceByOperator implementation

Validate FlinkReduceByOperator implementation

it is required to validate the implementation of FlinkReduceByOperator

because trigger an exception in the test and looks like is a problem in the

implementation of the implementation in the operator

labels:flink,bug

https://github.com/apache/incubator-wayang/blob/be9c4138fc6971401ce98d83125a753098134766/wayang-platforms/wayang-flink/code/test/java/org/apache/wayang/flink/operators/FlinkReduceByOperatorTest.java#L43

```java

* Test suite for {@link FlinkReduceByOperator}.
*/
public class FlinkReduceByOperatorTest extends FlinkOperatorTestBase{

//TODO: Validate FlinkReduceByOperator implementation
// it is required to validate the implementation of FlinkReduceByOperator
// because trigger an exception in the test and looks like is a problem in the
// implementation of the implementation in the operator
// labels:flink,bug
@Test
@Disabled("until validation of implementation of the FlinkReduceByOperator")
public void testExecution() throws Exception {
// Prepare test data.
List> inputList = Arrays.stream("aaabbccccdeefff".split(""))

```

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