Navigate to dashboard page if extension ID not found

[dsevillamartin]

Fixes #2583

Changes proposed in this pull request:

• Navigate to dashboard route if this.extension is undefined
• Return '' and null in className and view to prevent accessing this.extension before the route changes

Reviewers should focus on:
Do we want to add/change the returns or add checks when this.extension is used? The user doesn't see the rendered view as it instantly goes to the Dashboard page, but it will still error because Mithril calls the method (but doesn't render it)

Confirmed

• Frontend changes: tested on a local Flarum installation.