Do not add empty tags with confirmKeys

(*Nouvelle pull request dans notre fork… :blush:*)

When the input has some text in it, pressing `<Enter>` (or any of the `confirmKeys`) will add the text as a tag:

![](http://i.imgur.com/Y5vKcUk.png)

But when the input is empty, it pressing any of the `confirmKeys` should keep the default behavior (and submit the form):

![](http://i.imgur.com/jCiHMzA.png)