[BUGFIX] Fix validation in form override settings

The FinisherOptionGenerator of the form extension
generates the data structure for the form setting
overrides on the fly. By doing so, it didn't make
a distinction between normal TCA fields and flexform
specific sections/containers and added the wrapper
"TCEforms" to sections as well, which broke the
validation in DataHandler for the fields inside the
container (E.g. the email field).

The patch checks if section is set to "true" and
adds the "TCEforms" key only for normal TCA field
configurations.

Unit tests have been added for the DataHandler
checkValue_flex_procInData_travDS method, which
show cases with and without the "TCEforms" key.

Also, since this works now, a PHP 8 warning
appeared. This is fixed now, too.

Resolves: #95441
Releases: main, 11.5
Change-Id: Ib12a0484bcbdc827664181ca6af89edfadee13d8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71490


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