[BUGFIX] Fix undefined key warning in EXT:core/TimeTracker with PHP8

Resolves: #96350
Releases: main, 11.5
Change-Id: I67e9fd68de038b84c83f7f12a465e6e37f918568
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72654


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>