[BUGFIX] Fix warning on array key updateReferenceIndex

Patch fixes warning on PHP 8.1 in extbase Backend class.

Resolves: #98149
Releases: main, 11.5
Change-Id: I6f32f67672332758cc02708d07481b93edb09b82
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75497


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>