If we call burning with only existing instance, then it will fail to
create any of them, and thus in the removal phase it won't have anything
to remove. Since calling luxi.SUBMIT_MULTIPLE_JOBS with an empty job set
is an error (and will raise an exception), this creates a very strange
error in burnin (which is unfortunately hidden by ExecJobSet()).

As such, we modify CommitQueue to return immediately if it has an empty
op queue.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>