Exists matching works on the presence or absence of a field in the JSON of the event.
The value of the field is not considered when evaluating a match.[1]
* Add sentinel to distinguish between missing fields and existing fields with None values
* Update event pattern matching tests to cover this specific use-case.
* Add integrated test to cover this scenario within a larger context.
[1]: https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-event-patterns-content-based-filtering.html
