DataSet Should not union a NULL row for AGG without GROUP BY clause.

Currently DataSetAggregateWithNullValuesRule will UINON a NULL row for non grouped aggregate query. when CountAggFunction support COUNT(FLINK-8325). the result will incorrect.
for example, if Tabble T1 has 3 records. when we run the follow SQL in DataSet:

SELECT COUNT(*) as cnt from Tab // cnt = 4(incorrect).