Return the default list of JSON URLs if the list is empty.
Return the default list of JSON URLs if the list is empty.

The problem was that for tryserver.chromium it was returning an empty list,
which is why update_default_builders did not do anything.

Author: rmistry@chromium.org

Review: https://codereview.appspot.com/7838047/

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra