Webhooks: default tracking event tags=[], metadata={}

If a tracking event doesn't contain tags or metadata, set the event
record fields to tags=[] or metadata={} to simplify checking values.

Closes #67

anymail/webhooks/postmark.py

@@ -89,7 +89,7 @@ class PostmarkTrackingWebhookView(PostmarkBaseWebhookView):
         try:
             tags = [esp_event['Tag']]
         except KeyError:
-            tags = None
+            tags = []
 
         return AnymailTrackingEvent(
             description=esp_event.get('Description', None),