[BUGFIX] Only list valid forms

Only list valid forms within the form manger and the form plugin.

Valid forms have an "identifier" and a "type" and the type must be "Form".

Resolves: #85132
Releases: master, 8.7
Change-Id: I2f473925b3edf839d5f965136b6b02030671350a
Reviewed-on: https://review.typo3.org/57108


Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: TYPO3com <no-reply@typo3.com>