code: public function insertRowBefore($rowNumber, $row = null)
      {
          if ($rowNumber > count($this->collection)) {
              $this->appendRow($row);
          } else {
              $singleRow = $row;
              if (!($row instanceof Row)) {
                  $singleRow = new Row($row);
              }
              array_splice($this->collection, $rowNumber, 0, '');
              $this->collection[$rowNumber] = $singleRow;
          }
      }
docs: Insert one row before specified position.
  
  @param int $rowNumber
  @param mixed $row
  @throws \ByJG\Serializer\Exception\InvalidArgumentException