[BUGFIX] Provide a default value in TemplateService to avoid null values

Resolves: #97744
Releases: main
Change-Id: I60f3424696f2b6b655922b6c377ce65ec139c877
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74829


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>