[BUGFIX] Correct inline foreign_unique with target type="group" handling

Using type="inline" and MM relations having type="group" (instead of
type="select") and foreign_unique defined results in a fatal since
group db relations have been resolved to an array in TcaGroup data
provider already.

Change-Id: I20d138e7622aafb097a11e60014f9e4398bae811
Resolves: #82104
Resolves: #84735
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/58421


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Cyril Janody <typo3@cjanody.com>
Tested-by: Cyril Janody <typo3@cjanody.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>