Represents a JSON `null`. `NullValue` is a sentinel, using an enum with only one value to represent the null value for the `Value` type union. A field of type `NullValue` with any value other than `0` is considered invalid. Most ProtoJSON serializers will emit a `Value` with a `null_value` set as a JSON `null` regardless of the integer value, and so will round trip to a `0` value.