[TASK] Prevent undefined array key warnings in ext:beuser

This change addresses several "undefined array key" issues that have
been identified by PsalmPHP (see issue #98321).

Resolves: #98347
Related: #98321
Releases: main, 11.5
Change-Id: Ibca701d77324a4b2fe1d4a416deba5758d1ebde2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75732


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>