ArgoEvents.publish attempts to make requests even when url is None

[emilyselwood]

We have been seeing a bunch of error messages at the ends of our steps like:

Unable to publish Argo Event (metaflow.ExportFlow.start): unknown url type: 'None'

These were happening at the same time as other errors so we spent a bunch of time figuring out what they meant. I think I've tracked down to the publish method in argo_events. We have not set the ARGO_EVENTS_WEBHOOK_URL so None is being passed along to the request which fails.

https://github.com/Netflix/metaflow/blob/39524f7cddf62946bc4f032cc050f305b2d9d595/metaflow/plugins/argo/argo_events.py#L125

My guess is a check could be added to the function that forces an early return if self._url is None, preventing it attempting to make a http request that will fail. I'd be happy to make that change if people agree that this is whats happening?