burning: fix handling of empty job sets

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>