Vectorization: fix VectorUDAF* suspect isNull checks

    private void iterateHasNullsRepeatingSelectionWithAggregationSelection(
      VectorAggregationBufferRow[] aggregationBufferSets,
      int aggregateIndex,
      <ValueType> value,
      int batchSize,
      int[] selection,
      boolean[] isNull) {
      
      for (int i=0; i < batchSize; ++i) {
        if (!isNull[selection[i]]) {
          Aggregation myagg = getCurrentAggregationBuffer(
            aggregationBufferSets, 
            aggregateIndex,
            i);
          myagg.sumValue(value);
        }
      }
    }