[BUGFIX] Avoid calling htmlspecialchars(null) in TextElement

TextElement::render() calls htmlspecialchars() to output the
item value. It may be that $itemValue is null instead of a
string at this place and feed null to htmlspecialchars() emits
a E_DEPRECATED PHP 8.1 error. Sanitize call.

Resolves: #95931
Releases: master
Change-Id: I0b2066e7161b2e12e3a4b39d3f9669938a04c9bb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72108


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/backend/Classes/Form/Element/TextElement.php

@@ -227,7 +227,7 @@ class TextElement extends AbstractFormElement
         $mainFieldHtml[] = '<div class="form-control-wrap"' . ($width ? ' style="max-width: ' . $width . 'px">' : '>');
         $mainFieldHtml[] =  '<div class="form-wizards-wrap">';
         $mainFieldHtml[] =      '<div class="form-wizards-element">';
-        $mainFieldHtml[] =          '<textarea ' . GeneralUtility::implodeAttributes($attributes, true) . '>' . htmlspecialchars($itemValue) . '</textarea>';
+        $mainFieldHtml[] =          '<textarea ' . GeneralUtility::implodeAttributes($attributes, true) . '>' . htmlspecialchars((string)$itemValue) . '</textarea>';
         $mainFieldHtml[] =      '</div>';
         if (!empty($valuePickerHtml) || !empty($fieldControlHtml)) {
             $mainFieldHtml[] =      '<div class="form-wizards-items-aside form-wizards-items-aside--field-control">';