Validate FlinkJoinOperator implementation

Validate FlinkJoinOperator implementation

it is required to validate the implementation of FlinkJoinOperator

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/FlinkJoinOperatorTest.java#L41

```java

import org.apache.wayang.core.types.DataUnitType;
import org.apache.wayang.flink.channels.DataSetChannel;
import org.junit.Assert;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Disabled;

import java.util.Arrays;
import java.util.List;

//problematic
/**
* Test suite for {@link FlinkJoinOperator}.
*/
public class FlinkJoinOperatorTest extends FlinkOperatorTestBase{

//TODO: Validate FlinkJoinOperator implementation
// it is required to validate the implementation of FlinkJoinOperator
// 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 FlinkJoinOperator")
public void testExecution() throws Exception {
// Prepare test data.
DataSetChannel.Instance input0 = this.createDataSetChannelInstance(Arrays.asList(

```

596268dc3046c6b005a62d8a5beb8fb91d767535

Start with wayang-platforms/wayang-flink/code/test/java/org/apache/wayang/flink/operators/FlinkJoinOperatorTest.java at the disabled testExecution method and inspect the referenced FlinkJoinOperator implementation. Run the test to reproduce the exception and determine the expected validation behavior. Done means the join test no longer needs to be disabled and passes against a validated implementation.

Written by the indexing model from the issue text.