Enum cases that are unit should have no associated value

[adam-fowler]

Union members which have a unit type should have no associated type when rendered as enums with associated types.

eg

public enum EventFilter: AWSEncodableShape & AWSDecodableShape, Sendable {
        case all
        case include([Event])
}

instead of

public enum EventFilter: AWSEncodableShape & AWSDecodableShape, Sendable {
        case all(Void)
        case include([Event])
}