worksheet: fix if cellValue does not exist (#1727)

The condition is FALSE if the cell does not exist in the flipped table,
but anyway, it is sent in to a method requiring 'string' type, causing
it to fail.

src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

@@ -1079,7 +1079,7 @@ private function writeCellString(XMLWriter $objWriter, string $mappedType, $cell
     {
         $objWriter->writeAttribute('t', $mappedType);
         if (!$cellValue instanceof RichText) {
-            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue]);
+            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue] ?? '');
         } else {
             $objWriter->writeElement('v', $pFlippedStringTable[$cellValue->getHashCode()]);
         }