Allow empty string to be passed to styleName.

I have a use case where I conditionally want to apply a style, e.g.

```
<div styleName={isReady : 'ready' : ''}>
  ...
</div>
```

Currently there's no way to accomplish this, because the plugin will explode looking for a CSS module that uses an empty string. Is there a way to short-circuit this case and give up?