null check if value is null before trying to convert into jtoken

consider the following test:
```c#
        [Test]
        public void ExtensionDataWithNullValueProperty()
        {
            string json = @"{
            'TaxRate': 0.125,
            'a':null
            }";

            var invoice = JsonConvert.DeserializeObject<ExtendedObject>(json);

            string result = JsonConvert.SerializeObject(invoice);

            Assert.AreEqual(@"{""TaxRate"":0.125,""a"":null}", result);
        }

        class ExtendedObject
        {
            [JsonExtensionData]
            private IDictionary<string, JToken> _additionalData;
        }
```

the code throws argument null exception from jtoken.fromobject
the issue resides in DefaultContractResolver line 485 where the check does not include a check if the value is null.

adding that null check resolves the failing tests, and seralization for such cases no longer throws argumentnullexception