function testExec_CountDistinctNullableColumn() {
  var data = getEmployeeDatasetWithNulls();
  var inputRelation = lf.proc.Relation.fromRows(
      dataGenerator.sampleEmployees.concat(data), [e.getName()]);
  return checkCalculationForRelation(
      inputRelation,
      lf.fn.count(lf.fn.distinct(e.hireDate)),
      dataGenerator.employeeGroundTruth.distinctHireDates.length,
      checkEquals);
}
Count on a distinct column ignores nulls returned from distinct.
@return {!IThenable}