[BUGFIX] Fix undefined array key warning in AdministrationRepository

Running PHP8 on "Indexing Statistics" page an undefined array key
warning was thrown.

This is fixed by adding a null coalescing operator.

Resolves: #94845
Releases: master
Change-Id: If0668f528b314dd158acf29b5b744e6e4d0dc24d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70495


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>