[BUGFIX] Fix backend user check in ProductionExceptionHandler

Uncaught Error: Call to a member function isBackendUserLoggedIn() on null
in […]/typo3/sysext/core/Classes/Error/ProductionExceptionHandler.php:103

Change-Id: I0052c2c1cd617282dbc65320933f8577038ab41c
Releases: master, 8.7
Resolves: #83867
Reviewed-on: https://review.typo3.org/55688


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>