[BUGFIX] Make $actionName argument in ExtensionService nullable

The method ExtensionService::getPluginNameByAction() was hardened in
This breaks previous functionality, where it was possible to call
ActionController::redirect() without an action given, which produces
perfectly valid urls.

Resolves: #90422
Releases: master
Change-Id: Ib43a7655c03301d0d358b56003ad45a757a9a0dd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63291


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>