[BUGFIX] Do not end up in workspace and preview for default requests

Default requests won't have a workspace id.
That will lead to TYPO3 defaults -99 id.
That in turn will lead to being in preview mode once a backend user is
provided within context.

We therefore cover the default and turn it into live workspace which
could be the expected default.

Resolves: #535

Author: DanielSiepmann

Resources/Core/Functional/Extensions/json_response/Classes/Middleware/BackendUserHandler.php

@@ -55,6 +55,7 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
             // Init backend user if found in database
             $backendUser = GeneralUtility::makeInstance(FrontendBackendUserAuthentication::class);
             $backendUser->user = $row;
+            $workspaceId = $workspaceId ?? 0;
             if ($workspaceId !== null) {
                 // Force backend user into given workspace, can be 0, too.
                 $backendUser->setTemporaryWorkspace($workspaceId);