[BUGFIX] Ensure correct return values in Workspace middleware

When setting config.disablePreviewNotification = 1 in workspaces
context, the preview middleware returns "null", although the method
signature says it always requires a string return value.

Resolves: #89959
Releases: master, 9.5
Change-Id: Icdef1891989db2869c2351adb07e6b4778d25e09
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62651


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Achim Fritz <af@achimfritz.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Achim Fritz <af@achimfritz.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>