fix(dashboard): Correctly redirect guests to login page when opening dashboard

[nickvergessen]

Message: OCA\Dashboard\Service\DashboardService::__construct(): Argument #2 ($userId) must be of type string, null given
File: /var/www/html/apps/dashboard/lib/Service/DashboardService.php
Line: 15

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[nickvergessen]

/backport to stable29

[nickvergessen]

Only a problem in master