Make argument of Factory.build() to be optional

[vunguyenhung]

Hi,
Firstly, this is a very useful package. I usually use this with Faker.js to create fake data, for example:

// todo.factory.ts
export const TodoFactory = Factory.makeFactory<Todo>({
    content: faker.lorem.words(7),
    completed: faker.random.boolean()
});

// todo.seed.ts
const todo = TodoFactory.build({}); // pass empty object to build a Todo with default field.
export const seedTodos = () => TodoModel.create(todo); // pass created Todo to Mongoose model create()

So, when using this with Faker.js, the needs to define override field on Factory.build() is sometimes redundant. Therefore I think it's better to make the argument of build() todo optional.