[BUGFIX] Sanitize calling htmlspecialchars(...) in TextareaViewHelper

htmlspecialchars(...) expects that first argument is a string,
otherwise triggering E_DEPRECATED.

This patch adds a string cast to ensure value is passed as string
to htmlspecialchars(...) in TextareaViewHelper.

Resolves: #95831
Releases: master
Change-Id: I5fe32233bed62ab8aaa8405ed8913232d5f82ab5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72013
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>

typo3/sysext/fluid/Classes/ViewHelpers/Form/TextareaViewHelper.php

@@ -72,7 +72,7 @@ public function render()
         if ($required === true) {
             $this->tag->addAttribute('required', 'required');
         }
-        $this->tag->setContent(htmlspecialchars($this->getValueAttribute()));
+        $this->tag->setContent(htmlspecialchars((string)$this->getValueAttribute()));
         $this->addAdditionalIdentityPropertiesIfNeeded();
         $this->setErrorClassAttribute();