[BUGFIX] Avoid undefined array key "parent." in AbstractPlugin

Resolves: #99085
Releases: main, 11.5
Change-Id: Ie3b337b93f43fb2c41d76427f29d793b57e9232b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76598


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>