Nulls serialization

[breslavsky]

Primitives with null values are not presented in serialized object

@Model()
class User {
  @Field()
  firstName: string;

  @Field()
  lastName: string;
}
let admin = new User();
admin.firstName = "Anton";
admin.lastName = null;

console.log(serialize(admin));
// {firstName: "Anton"}
// but expected
// {firstName: "Anton", lastName: null}

Question is here https://github.com/AndreyZelenskiy/serialize/blob/master/src/serializers/field.utils.ts

export function isPresent(obj: Object): boolean {
  return obj !== undefined && obj !== null;
}

Perhaps remove obj !== null ?