[BUGFIX] Resolve PHP 8 issues in acceptance tests

PHP 8 promotes a lot of notices (previously ignored) to warnings.
That means they can no longer be ignored.

Resolves: #94057
Releases: master
Change-Id: I37de1b7e8e31eb2be78b8e90af015bc88e121791
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69012


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>