Empty hash passed as attributeBindings should not make any effects

Currently when specifying options for ModelBinder.bind method we need to pass null for attributeBindings argument.

```javascript
modelBinder = new Backbone.ModelBinder()
modelBinder.bind(model, el, null, {modelSetOptions: {silent: true}})
```

It's very common to pass empty hash if you want to skip values for first options argument. But then ModelBinder will not initialize default bindings since {} is a true value.

```javascript
modelBinder = new Backbone.ModelBinder()
modelBinder.bind(model, el, {}, {modelSetOptions: {silent: true}})
```

This pull request fixes second case by checking whether attributeBindings is an empty object or null.