[BUGFIX] EXT:form - "label" is not set for the FormDefinition class

Set the "label" property for the root form element
TYPO3\CMS\Form\Domain\Model\FormDefinition

Resolves: #82573
Releases: master,8.7
Change-Id: I57c45765e4af91da6094f8f7a469a1df2f6da1f5
Reviewed-on: https://review.typo3.org/54227


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Sonja Großewinkelmann <s.grossewinkelmann@neusta.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>