[TASK] Ensure TSFE is really a valid object

Make sure the TSFE fetched in TemplateService really is a valid
TypoScriptFrontendController.
This way any kind of "fake Frontend" built in Backend does not
break the functionality.

Resolves: #91139
Releases: master
Change-Id: I571130de635f2be79301f747c5b3e1ad40987fec
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64255


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>