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).
