event merger: Handle null Update objects

With the refactor for late-fetch from the consumer the inputs to the event merger have changed. Where before all events would have an Update to be merged, now events that still need to be fetched will have a null Update object. Take that into account, and add tests that invoke this functionality.

The update_pipeline schema had to be changed, because we are now emitting updates that contain only the weighted_tags and no other fields. The required fields could be constucted, but we don't necessarily want to push them as an update to elastic so they have been removed from the required properties list.

Bug: T347075