Consider allowing nil data

[mostlyobvious]

At the moment there's following column specification in event_store_events:

t.binary      :data,        null: false

When there's no data in an event, we'll store serialized (empty) hash. As an optimization we might store nothing (when null is allowed).

Aside: are events without data useful? I think so. Publishing events is usually a good first step to gradually convert to DDD/ES/CQRS way of thinking. Events without data might also come from and Event Storming session when you're prototyping.