[BUGFIX] Add fallback for "task_group" in SchedulerModuleController

When creating a new task using PHP8 a undefined key warning
is thrown.

To fix this issue a fallback for "task_group" has been added.
Also extended createASchedulerTask test to check for
expected output in case nom settings (defaults) are given.

Resolves: #95088
Releases: master
Change-Id: I47a480e635eaffbad2e1a1d03426a7acc04cc494
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70871


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>