During encoding with `encodeIfPresent`, what is emitted for a nil Optional?
struct Settings: Encodable {
var theme: String?
func encode(to encoder: Encoder) throws {
var c = encoder.container(keyedBy: CodingKeys.self)
try c.encodeIfPresent(theme, forKey: .theme)
}
enum CodingKeys: String, CodingKey { case theme }
}
// theme == nil