Avoid error on CORS requests

[Logianer]

Made constructor parameter $userId of type ?string (= be optional)
If no userId is passed (aka. is null),
$this->userId is set to an empty string to not cause any problems or errors.

See #465 for more info

[juliusknorr]

Thanks a lot for your pull request. Your change mages sense, though the fallback to empty string is not required and we should probably just keep the null value then. Could you adjust that? Then we can get this merged for the next release

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[datenangebot]

Makes sense. But null is better than '' because an empty string is sometimes used as 'super admin' with additional permissions.

Failing test belongs to #453.
Will merge.

@Logianer Many thanks.