[BUGFIX] Avoid undefined array key access in StaticRouteResolver

Resolves: #101394
Releases: main, 12.4, 11.5
Change-Id: I1d0cbb99496108180cf1f06045934bb510abcd3f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80150


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>