[BUGFIX] Filelist folders not expandable

A wrong type given to JsonResponse->setPayload()
fatals the return type on expanding the filelist
folder tree.

Change-Id: I7510ba7d5098b8b91c5016472487f73c6ca77e2c
Resolves: #83303
Releases: master
Reviewed-on: https://review.typo3.org/55053


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>