@Override
  public Object getGroupResult() {
    return firstValue;
  }
This class evaluates double first_value() for a PTF group.

We capture the first value from the first batch.  It can be NULL.
We then set (stream) the output column with that value as repeated in each batch.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/ptf/VectorPTFEvaluatorDoubleFirstValue.java