@Override
  public boolean isGroupResultNull() {
    return isGroupResultNull;
  }
This class evaluates double first_value() for a PTF group.
We capture the last value from the last batch.  It can be NULL.
It becomes the group value.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorDoubleLastValue.java