[BUGFIX] Prevent undefined array key warning in LinkService

Used command(s):

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #100565
Releases: 11.5
Change-Id: I11d1680ec707e7c93d8dcbfcf558d840094a1a65
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78692


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Sybille Peters <sypets@gmx.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>