[BUGFIX] Avoid error for orphaned ts templates

When a page containing a typoscript template has been
deleted, there won't be a valid rootline for the template.
Add a simple check to avoid an "undefined array key" error.

Resolves: #100365
Releases: main
Change-Id: Ic082c9b1d895241b87a372e490031626b91b1604
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78338


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>