code: public long rowsWithNa() {
      if( _rowsWithNa!=-1 ) return _rowsWithNa;
      String x = String.format("(na.omit %s)", _fr._key);
      Val res = Rapids.exec(x);
      Frame f = res.getFrame();
      long cnt = _fr.numRows()  -  f.numRows();
      f.delete();
      return (_rowsWithNa=cnt);
  
    }
docs: count of rows with nas