jqueue: Allow zero jobs to be submitted at once

If cmdlib.LUNodeMigrate was called for a node without primary instances
it would try to submit an empty list of jobs. This was never visible via
CLI as there we check the list of primary instances first.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>