[BUGFIX] Avoid 'Undefined array key' warning in LinkValidator

Resolves: #96786
Releases: main, 11.5
Change-Id: I69742ee10a6704cd43f8c68733bf13302200b975
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73356


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>