fix(notifications): Fix check for hasNotifiers when all apps use Regi…

[nickvergessen]

…strationContext

• Resolves: Red CI in https://github.com/nextcloud/notifications/actions/runs/15432761760/job/43433471031
• This is triggered by refactor(core): migrate core application to IBootstrap #53157 which removed the last non-RegistrationContext registered Notifier that was always there. After that pull request there was never a direct one and the RegistrationContext was not yet tested whether it really has any notifiers. This caused hasNotifiers() to return false even though it had notifiers later on when actually getting them. But the notifications app did an early exit with a better status code when non notifier exists.

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 stable31

[nickvergessen]

/backport to stable30