Cannot read property 'icons' error fixed

Hey!

I've found a bug that happens when you click on a emojis groups button. That happens because clicking the a element of the menu the emojiarea.js was trying to get the emoji of that anchor element, and emojiarea was trying to find an undefined emoji and an empty string "" group.

Error text:
```
Uncaught TypeError: Cannot read property 'icons' of undefined   jquery.emojiarea.js:197
```

I've fixed it just with a simple if conditional that checks if emoji isn't undefined and group isn't an empty string.

Btw, great job putting all the emojis by groups! You've saved my day!