[BUGFIX] Avoid undefined array key in ObjectStorage::offsetGet()

Requesting an none existing entry resulted in an PHP Warning in newer
PHP 8.x versions.

Resolves: #97554
Releases: main, 11.5
Change-Id: Ie0096b8e2fe6593aa577279d2ef843151d1af8b8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77263


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