[BUGFIX] Replace misleading instanceof check for notification actions

With #95406, an instance of check was added to the
Notification component, to prevent a TypeError
in case the action is not defined.
That was actually a sign of a bad usage (e.g. passing null),
and therefore the action is now made optional instead.

Resolves: #95420
Related: #95406
Releases: master
Change-Id: I52c519817225f0a874788095a912c808528d031d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71388


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>