Improvements to bundling aggregations. (#11815)

This is some odds and ends found during the review of #11791
and while continuing to work in this code:

* Return attrs classes instead of dictionaries from some methods
  to improve type safety.
* Call `get_bundled_aggregations` fewer times.
* Adds a missing assertion in the tests.
* Do not return empty bundled aggregations for an event (preferring
  to not include the bundle at all, as the docstring states).

synapse/push/mailer.py

@@ -455,7 +455,7 @@ class Mailer:
         }
 
         the_events = await filter_events_for_client(
-            self.storage, user_id, results["events_before"]
+            self.storage, user_id, results.events_before
         )
         the_events.append(notif_event)