Don't set build.started_at and build.finished_at as default

Because of chages of a153d78, spec/models/job_spec.rb fails.

Job#current_build returns builds that don't have finished_at.
So if build.finished_at is set, current_build returns nil and spec fails.
Set build.finished_at to have multiple builds

In 9d1a92, I've made the change not to set started_at and finished at.
But to have multiple builds, build must have finished_at attribute.
Return if Altria::Executer.execute returns nil

In the example "Job#run with script runs its job", Altria::Executer.execute returns
nil and this example fails.So I've made this fix.
(I don't know why Altria::Executer.execute returns nil.)