contextualizeBindingPath should be aware of empty paths

Binding a property to `this` keyword when calling a helper created with `Ember.Handlebars.helper`, make 
[contextualizeBindingPath](https://github.com/emberjs/ember.js/blob/a316fa4bbd335b08b6ff3b9145e340bc64003fa0/packages/ember-handlebars/lib/helpers/view.js#L145), fallback in `'_parentView.context.' + path` but the path variable is empty and this result in a wrong path `_parentView.context.` (trailing dot). So, we need to check the empty value. Because handlebars generate `{ hash: { model: '' }, hasContext: { model: <instance> } }` when `model=this` is specified in the template.

close #3860

//cc @machty