Empty key is treated as not exsisting

Hi,

When I add empty key:

```
i18n.map('en', {
    key: ""
});
```

with option `i18n.showMissing(true);` the handlebars helper `{{i18n 'key'}}` is returning the key not empty string. Same goes to the `i18n` function. I think it's bug; and I just don't have time to write fix and make pull request on my own :)

Cheers
