facebook/react — pull request #1682 by sbrekken (2014-06-12 07:39:12+00:00)
Title: Reject falsey cx() arguments

This adds another layer of convenience to `cx` by rejecting falsey arguments.

Example use:

```javascript
var active = true
var empty = ''
var classes = cx('product-list', active && 'active', emptyClass)
```

Yields:

```javascript
"product-list active"
```