webhook: Accept unregistered jobs, assume thye don't have to wait

The current code requires a pre-flight registration of the submitted jobs before making use of the data tha gets posted by the resulting LAVA callback.

The reason is that the current code tries to coalesce multiple jobs submitted for the same image version so it can provide a summary if the same error happens on multiple architectures or multiple image types.

However during testing this is cumbersome, so let's just assume there's no need to wait for other jobs if we get a job completion callback with no pre-registration.