[BUGFIX] Fix undefined array key in TemplateService

If a TypoScript sub-template is missing, because it is disabled,
it is necessary to check for the existence.

Resolves: #97601
Releases: main, 11.5
Change-Id: Iaca27df294f4c3c3ea890defc4ccd3b5b6feb671
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74612


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>