Fix script error on some custom regex.

I'm not sure if this should be merge, this a bug I have but I'm not sure is this come from me (some error in my Regex) or if this a "normal" behaviour. 

In some case, my custom regex used to extract the translation have some strange behaviour: 
```javascript
i18next({
    locales: ['en', 'fr'],
    parser: 'this\\.env_\\.filter\\("trans", "(.*)"\\)|,',
    output: '../build/statics/translations/'
});
```
The result is on lines https://github.com/karellm/i18next-parser/blob/282e69c566154bc251b1e3cc3515b1096cd97a94/index.js#L102-L104  the variable `matches` are fill with an array but with an undefined entry 1:
```javascript
console.log(typeof matches[1] == 'undefined') // true
```

So this patch have to aim to prevent this case.