JSONDecoder...was a null value present?

When sending a JSON payload, sometimes it's important to know whether a null value was sent, especially for a PATCH type operation. The struct that I decode to will obviously have that as a nullable type. However, I can't see any way to know whether it's nil because the JSON contained the key with a null value, or whether it's nil because the JSON just didn't send that key. How do you handle this?