feat(tags): Allow overriding default tag
feat(tags): Allow overriding default tag

Because "no tags" is `None` and "no preference" is `one`, we need to
allow `[]` to also represent `no tags` and convert it to `None`.

cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs