Handling nullable

I have an instance where I am trying to use SmartEnumValueConverter but since the value can be null it throws an error rather than settings the SmartType to null. How can this be handled?

Error: Newtonsoft.Json.JsonSerializationException : Error converting Null to SmartType.

Example:
``` json
{
type: null
}
```

Type can be null, 0, 1, 2, etc.

``` csharp
[JsonConverter(typeof(SmartEnumValueConverter))]
public SmartType? Type { get; set; }
```

Start at the SmartEnumValueConverter entry point and reproduce the Newtonsoft.Json deserialization case using the nullable SmartType property and the JSON example in the issue. Done means a null value deserializes without an exception and sets Type to null, while numeric values such as 0, 1, and 2 continue to work.

Written by the indexing model from the issue text.