fix: Allow guests to open public pages again

[nickvergessen]

Steps

1. Create a public form
2. Open it as a guest
3. :boom:

Cannot assign null to property OCA\Forms\Service\ConfigService::$currentUser of type OCP\IUser

// After fixing that
Cannot assign null to property OCA\Forms\Activity\ActivityManager::$currentUser of type OCP\IUser

Not sure if you want to apply some hardening to prevent methods from calling things on currentUser which can be null now, so please take over @susnux

[nickvergessen]

Fixed via #2293