code: public function isValid(Cell $cell)
      {
          if (!$cell->hasDataValidation()) {
              return true;
          }
  
          $cellValue = $cell->getValue();
          $dataValidation = $cell->getDataValidation();
  
          if (!$dataValidation->getAllowBlank() && ($cellValue === null || $cellValue === '')) {
              return false;
          }
  
          // TODO: write check on all cases
          switch ($dataValidation->getType()) {
              case DataValidation::TYPE_LIST:
                  return $this->isValueInList($cell);
          }
  
          return false;
      }
docs: Does this cell contain valid value?
  
  @param Cell $cell Cell to check the value
  
  @return bool