scheduler: Update default policy to schedule origins without last update

Prior to this, we considered not to do it. However, we do have some listers which are not able to list origins with a last update. And we still need to be able to schedule those origins nonetheless hence this change.

Refs. swh/infra/sysadm-environment#4971 (closed)

mentioned in merge request swh-environment!266 (merged)

approved this merge request

Thanks a lot!

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merged