[BUGFIX] Set nullable typehint within setBackendUserAspect()

Use a nullable typehint for the method setBackendUserAspect() within
TYPO3\CMS\Frontend\Middleware\TypoScriptFrontendInitialization to avoid
PHP fatal errors if the backend user does not have backend access rights
for a frontend page.

Resolves: #89763
Releases: master
Change-Id: I076c34be2fa7b35d4bcdefb13a6b3f385e96b026
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62424


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>