[BUGFIX] Only process arrays in BackendUserConfigurationUpdate

A check is added to assure the unserialized content of
a be_users uc field is actually an array before passing it to
array_walk_recursive(). This is needed because the uc field
can also contain null.

Resolves: #89638
Related: #89268
Releases: master, 9.5
Change-Id: I9a3d56525b9434522e719d88300240abe9c9c5be
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62368


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>