[BUGFIX] Add type check to prevent error if backend user is null

$backendUser may be null, this would cause an error. Add an
additional type check to prevent this.

Resolves: #78392
Releases: master
Change-Id: Ifa2b2f30283f5740a60edddb5670be9291706f90
Reviewed-on: https://review.typo3.org/50337


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>