Azure Event Hubs client library samples for JavaScript

[event-hubs] doc improvements (#9342)

* [event-hubs] explain empty events

* [event-hubs] readme update for 8798

* [event-hubs] migration-guide checkpointing 8419

* update changelog for 5.2.1

* Update sdk/eventhub/event-hubs/migrationguide.md

Co-authored-by: Richard Park <51494936+richardpark-msft@users.noreply.github.com>

* Decided to be more ambitious for the release date

Co-authored-by: Richard Park <51494936+richardpark-msft@users.noreply.github.com>