private static void propagateNulls(boolean selectedInUse, int n, int[] sel, ColumnVector inV,
      ColumnVector outV) {
    if (selectedInUse) {
      for(int j = 0; j != n; j++) {
        int i = sel[j];
        outV.isNull[i] = inV.isNull[i];
      }
    } else {
      System.arraycopy(inV.isNull, 0, outV.isNull, 0, n);
    }
  }
Propagate nulls from input vector inV to output vector outV.

@param selectedInUse true/false flag to tell if sel[] is in use
@param sel selected value position array
@param n number of qualifying rows
@param inV input vector
@param outV output vector
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringGroupConcatColCol.java