@Override
  public String aggregateStats(String keyPrefix, String statType) {
    return null;
  }
An test implementation for StatsAggregator.
The method corresponding to the configuration parameter
hive.test.dummystats.aggregator fail, whereas all
other methods succeed.
itests/util/src/main/java/org/apache/hadoop/hive/ql/stats/DummyStatsAggregator.java