Set AnymailTrackingEvent.metadata to empty dict instead of None

[lauritzen]

If there is no metadata attached a tracking event the metadata field is set to None. This is a bit inconvenient while handling tracking events as it introduces an extra conditional before accessing the datastructure:

if event.metadata:
  value = event.metadata.get(key)

Setting metadata = {} instead removes the need for the conditional.

[medmunds]

That seems reasonable. I think my intent was for None to indicate the ESP hadn't included metadata in the webhook, so we couldn't be sure whether that meant the message was sent without metadata, or whether the ESP simply didn't pass it along with the event. But looking at the implementation, I don't think it ended up working that way.

I wonder if tags should also be initialized to [] rather than None, for similar reasons.

[lauritzen]

Great. I agree on the same approach for tags.