fix(model)!: incoming heartbeat has null data

[vilgotf]

My shard received a heartbeat event and apparently we model that wrongly. Discord documents this event as containing data, but that's only when sent to the gateway.

2025-02-14T15:50:16.796316Z DEBUG twilight_gateway::shard: received heartbeat
2025-02-14T15:50:16.796359Z  WARN gateway_request_members: error receiving event source=ReceiveMessageError { kind: Deserializing { event: "{\"t\":null,\"s\":null,\"op\":1,\"d\":null}" }, source: Some(Error("invalid type: null, expected u64", line: 1, column: 34)) }

[Erk-]

Going to change the name to include a ! since we want to mark this as a breaking change in the change log.