[BUGFIX] Avoid PHP 8.0 undefined key in FormEngine

FlexFormSectionContainer raises an exception when
title is not defined.

Releases: main, 11.5
Resolves: #96936
Change-Id: I0ad1a432f11bb2cc36f718ca25f430cf91d94417
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73538


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