[BUGFIX] Avoid calling htmlspecialchars(null) in GeneralUtility

GeneralUtility::implodeAttributes() calls htmlspecialchars() on the
array values to sanitize them. The values may be null instead of a
string, thus feeding null to htmlspecialchars() emits a E_DEPRECATED
PHP 8.1 error. Sanitize call.

Resolves: #95933
Releases: master
Change-Id: I57f1cdf2eb2e818283f5e1c1305ce4d2f3b9d440
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72110


Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>