[BUGFIX] Fix PHP warnings on fe_user log out

PHP Warning: Illegal string offset is thrown in some places
if an fe_user logs out.

Resolves: #55696
Releases: 6.2
Change-Id: I532a27c0d2fc4b80d3a3f92da0079f4e9517d06c
Reviewed-on: https://review.typo3.org/27362
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Oliver Klee
Reviewed-by: Alexander Opitz
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind