[BUGFIX] Prevent undefined array key warning in CleanFlexFormsCommand

Resolves: #97619
Releases: main, 11.5
Change-Id: Ib1812d04264cab9bf867891f8cc06d7dafeace99
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74617


Tested-by: core-ci <typo3@b13.com>
Tested-by: Alexander Grein <alexander.grein@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Alexander Grein <alexander.grein@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>