[BUGFIX] Properly initialize backend user for 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.

Also we add missing initialization for backend user.

Resolves: #535

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

@@ -55,6 +55,10 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
             // Init backend user if found in database
             $backendUser = GeneralUtility::makeInstance(FrontendBackendUserAuthentication::class);
             $backendUser->user = $row;
+            $backendUser->uc = isset($row['uc']) ? unserialize($row['uc']) : [];
+            $backendUser->initializeUserSessionManager();
+            $backendUser->fetchGroupData();
+            $workspaceId = $workspaceId ?? 0;
             if ($workspaceId !== null) {
                 // Force backend user into given workspace, can be 0, too.
                 $backendUser->setTemporaryWorkspace($workspaceId);