[BUGFIX] Prevent PHP warnings in workspace ActionHandler

* $parameters->additional might not be set,
   depending on the workspace configuration
* There might be no "next" or "prev" stage,
   depending on the current stage

Resolves: #97368
Releases: main, 11.5
Change-Id: Ic1a5452adb33f900bc2c8c0c86ee1d5d9849d290
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74296


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>