code: protected function adjustForExistingOnlyRange()
      {
          if ($this->onlyExistingCells) {
              while ((!$this->worksheet->cellExistsByColumnAndRow($this->startColumnIndex, $this->rowIndex)) && ($this->startColumnIndex <= $this->endColumnIndex)) {
                  ++$this->startColumnIndex;
              }
              if ($this->startColumnIndex > $this->endColumnIndex) {
                  throw new PhpSpreadsheetException('No cells exist within the specified range');
              }
              while ((!$this->worksheet->cellExistsByColumnAndRow($this->endColumnIndex, $this->rowIndex)) && ($this->endColumnIndex >= $this->startColumnIndex)) {
                  --$this->endColumnIndex;
              }
              if ($this->endColumnIndex < $this->startColumnIndex) {
                  throw new PhpSpreadsheetException('No cells exist within the specified range');
              }
          }
      }
docs: Validate start/end values for "IterateOnlyExistingCells" mode, and adjust if necessary.
  
  @throws PhpSpreadsheetException