[push-api] Make `PushEvent.data` nullable (#161)

Per discussion with @martinthomson, once encryption lands, we'll want a way to distinguish messages without an entity body from encrypted, blank messages. For the former, it seems sensible to set `PushEvent.data` to null.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/161