[BUGFIX] EXT:form - parse finisher values only if string

Parse finisher option values only if the referenced formelement value
is a string.

Resolves: #79412
Releases: master
Change-Id: Ie379a396473c31907d52c8aa096168fed803510b
Reviewed-on: https://review.typo3.org/51381


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Lorenz <daniel.lorenz@extco.de>
Tested-by: Daniel Lorenz <daniel.lorenz@extco.de>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>