sync_events: Make all fields in the response optional except next_batch.
sync_events: Make all fields in the response optional except next_batch.

As per spec only the next_batch token is required in a sync response.

This patch makes all of them optional and creates default values when
parsing a response. Serialization is skipped for all of them if they are
empty containers.

ruma/ruma,ruma/ruma-client-api