Using `isEmpty` to check validity of `render` options breaks `openAt` functionality

[akrawchyk]

When rendering a modal, you have the ability to pass in options to the render function: https://github.com/awkward/backbone.modal/blob/master/src/backbone.modal.coffee#L23.

Then, these options are tested for existence and emptiness https://github.com/awkward/backbone.modal/blob/master/src/backbone.modal.coffee#L26.

Then, later in the render function, these same options are passed to openAt https://github.com/awkward/backbone.modal/blob/master/src/backbone.modal.coffee#L42.

Consider this example, please assume valid HTML templates for everything. This does not work due to _.isEmpty returning false for the value 1 passed to render. We end up rendering the AddView since options is assigned 0 here https://github.com/awkward/backbone.modal/blob/master/src/backbone.modal.coffee#L26:

var AddView = Backbone.View.extend({ ... });
var EditView = Backbone.View.extend({ ... });

var Modal = Backbone.Modal.extend({
    template: _.template($('#js-modal-template').html()),
    viewContainer: '.bbm-modal__section',
    views: {
        'add': {
            view: AddView
        },
        'edit': {
            view: EditView
        }
    }
});

var modal = new Modal();

// Fails to render modal with `EditView` (e.g. views[1]) triggered, instead triggers `AddView` (e.g. views[0])
$('.edit').on('click', function() {
    $('.modal).html(modal.render(1).el);
});

We can achieve the desired functionality two ways with small modifications to the render call, either by chaining a call to openAt or by passing in an object with an _index key (https://github.com/awkward/backbone.modal/blob/master/src/backbone.modal.coffee#L293) to the call to render:

// Workaround 1
$('.edit').on('click', function() {
    $('.modal).html(modal.render().openAt(1).el);
});

// Workaround 2
$('.edit').on('click', function() {
    $('.modal).html(modal.render({ _index: 1 }).el);
});

The first is less efficient since we are making 2 calls to openAt, once as openAt(0) within the render function, and a second time with the chained call to openAt(1).

The second is OK, just undocumented. Also, the _index key indicates to me it is private which means it could change internally and is therefore unreliable.