[BUGFIX] Prevent undefined array key warning in FormPersistenceManager

Resolves: #97199
Releases: main, 11.5
Change-Id: Ibb65dbccbf26b0522ce1c8ccf8c78a1cea29e9c6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73958


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>