Fix #duration when #scheduled_at is nil

[ric2b]

Problem

When a job is enqueued without scheduled_at being set the app/views/mission_control/jobs/jobs/show.html.erb template breaks as the duration calculation currently expects scheduled_at to have a value.

Solution

Calculate the duration with enqueued_at if scheduled_at is nil.

Fixes #282

[ric2b]

Looks like this might not be necessary after SolidQueue 1.2.2 was released? It includes this fix: rails/solid_queue#637

Although I'm not sure if ActiveJob expects every job to have scheduled_at populated or not, this might still be an issue in other backends if it's not mandatory.

[rosa]

I think this is a good idea regardless! Thank you.