[BUGFIX] Fix warning when FlexForm sheet has no fields

This patch fixes a warning on PHP 8.1, when a FlexForm has
a sheet with no fields in it.

Resolves: #98166
Releases: main, 11.5
Change-Id: Iec814a10bb755fb78a8de95b432d8cb6042a07d3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75518


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