Fix a problem that callback isn't called if options empty

I got a problem like this:
```
monky.factory('User', {});

monky.build('User', function(err, user){
  // Never here is executed.
});
```

It was troubled when I test the document that applies the default value of the mongoose side:
```
var userSchema = new mongoose.Schema({
  name:
    type: String
    default: 'Default Name'
});
```