[BUGFIX] Normalize the `if` and `bind-attr` truthiness

There exists some inconsistent in evaluating the truthiness of an empty array between the `{{#if}}` and `{{bind-attr}}` helpers. The former treats an empty array as `falsy` value (still i'm not sure [why handlebars {{#if}} treats an empty array as falsy](https://github.com/wycats/handlebars.js/pull/66)) while the latter treats it as `truthy` one.

[JSbin demonstrating the inconsistent behaviour](http://emberjs.jsbin.com/pagawife/2/edit)
[JSbin fixing the inconsistent](http://emberjs.jsbin.com/tofisetu/1/edit)

This PR addresses the inconsistent behaviour.
Fixes #4330

/cc @wagenet