[BUGFIX] Avoid calling strlen(null) in TextElement

TextElement::render() calls strlen() to determine length of form
elemet item value. It may be that item value is null instead of a
string at this place and feed null to strlen() emits a E_DEPRECATED
PHP 8.1 error. Sanitize call.

Resolves: #95930
Releases: master
Change-Id: Ib7f073d59e4bbc39fb070d86bbfe3a5b69d3bef8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72107


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>