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

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

Resolves: #98325
Releases: main, 11.5
Change-Id: I340f49059677c05348a3d4fedc2aa4ed0aca843b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75703


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>