[BB-2450] Fix server error

Created by: 0x29a

This is a follow-up PR with fix for https://github.com/open-craft/opencraft/pull/591. It adds fix for notifications endpoint and message for Status & Notification page when user has no instance.

JIRA tickets:

• BB-2450

Dependencies: None

Screenshots:

Testing instructions:

1. Register, but don't verify email.
2. Go to Status & Notifications page.
3. You should see message like on screenshot.

Reviewers

• @Agrendalath
• @xitij2000