[fix]: allow other scopes than system but provide completion for system scope

[foxriver76]

Link the issue which is closed by this PR

• closes #2731

Implementation details

We extended the definition of the NotificationScope to allow other unknown scopes with any category as long as type is string | null

Tests

• [x] I have added tests to avoid a recursion of this bug
• [ ] It is not possible to test for this bug