[BUGFIX] Avoid calling htmlspecialchars(null) in InputTextElement

InputTextElement::render() calls htmlspecialchars() to output the
itemValue. $itemValue can be null null instead of a string at this
place and feeding null to htmlspecialchars() emits a E_DEPRECATED
PHP 8.1 error. Sanitize call.

Resolves: #95929
Releases: master
Change-Id: I96eeb6fb0b19e5f751b68e835a59726c7a4657b6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72106


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