Event.toString() throws NPE if context is null

Suggested release note:

If you were subscribing to the event bus and printing all the events, you may have noticed `Event.toString()` throwing a `NullPointerException` if the event context is null. `Event.toString()` is now handles null contexts more gracefully, and no longer throws this exception.

Event context is de-facto nullable, but Event.toString() always dereferences it.