[TASK] Make the action name in ActionController::redirect() nullable

Passing no action name is a perfectly fine use case as this allows
to conveniently redirect from a controller action to any page.

Making this explicit in the type annotations allows extension authors
to use this pattern without type checkers like PHPStan creating a
warnings about this.

Resolves: #99110
Releases: main, 11.5
Change-Id: I12cded81cbe20f7f2f626cf71ab92532d9037ee9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76654


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>