Generate null checking code if the property is nullable when using `@JsonKey(fromJson)`

Let's consider this example.
```
@JsonSerializable()
class Product {

@JsonKey(fromJson: ProductType.byId)
final ProductType? type;

const Product({required this.type});

}
```

Currently when I run `build_runner`. The generated code will look like this.
```
Product _$PFromJson(Map json) => Product(
type: ProductType.byId(json['type'] as String),
);
```

I'm suggesting if the property is nullable, then the generator can set a null value for the type property.
```
Product _$PFromJson(Map json) => Product(
type: json['type'] == null ? null : ProductType.byId(json['type'] as String),
);
```