In case the adminOnly flag is set, an information window is shown.
The middleware now creates a valid JSON response to show
the information window again.

Resolves: #84765
Releases: master
Change-Id: Ibe36b9d2c06b730bb2687d528d011268af4da660
Reviewed-on: https://review.typo3.org/56706


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>