[BUGFIX] Set fallback for undefined array key in php8

After reset user preferences in settings module the
Configuration Module shows undefined array key warning.
An empty array has been added as fallback to workaround
this issue and improved scrollTo behavior when the user
opens a node in the configuration tree.

Also extended tests to cover such issues in the future.

Resolves: #94487
Releases: master
Change-Id: I3e907a2618765d314ff1ad8bcab74f1af46765c0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69724


Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: crell <larry@garfieldtech.com>
Reviewed-by: Benni Mack <benni@typo3.org>