ORC: validate values are not null for required columns

[rdblue]

The SparkOrcWriter defines converters that could easily throw exceptions when a required column has a null value, like this:

  static class RequiredIntConverter implements Converter {
    public void addValue(int rowId, int column, SpecializedGetters data,
                         ColumnVector output) {
      if (data.isNullAt(column)) {
        throw new NullPointerException("Column " + column + " is required, but null in row: " + rowId);
      } else {
        output.isNull[rowId] = false;
        ((LongColumnVector) output).vector[rowId] = data.getInt(column);
      }
    }
  }