[BUGFIX] Array access warning in DataHandler

This may be triggered by invalid FlexForm values
that does not contain a 'data' key for whatever
reason.

Resolves: #100733
Releases: main, 12.4, 11.5
Change-Id: I96924d65af56fd7cf2bb2a97fd4591b3c81f934f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81586


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>