[BUGFIX] Correctly handle validation of multi-selects in FormEngine

Resolves: #72366
Releases: master, 7.6
Change-Id: I7627a2a5b1aec4428d160e6b201373b8cef779ee
Reviewed-on: https://review.typo3.org/45393


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>