[BUGFIX] Avoid calling substr_count(null) in TemplateService

TemplateService::render() calls substr_count() on a array field
retrieved from the database, which may be null instead of a string.
Feeding null to substr_count() emits a E_DEPRECATED PHP 8.1 error.
Sanitize call.

Resolves: #95932
Releases: master
Change-Id: I1429d0f49356922208acf9ccb2ab8b8bbd53043c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72109


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>