Which sequence preserves the distinction between an absent key and an explicit JSON null?

Which sequence preserves the distinction between an absent key and an explicit JSON null?

1. Check contains(_:), then decodeNil(forKey:), then decode(_:forKey:).
2. Call decodeIfPresent(_:forKey:) once and inspect whether the result is nil.
3. Give the Swift property a declaration default and use synthesized decoding.