Prevent empty slugs.

You said [here](https://github.com/cviebrock/eloquent-sluggable/issues/21#issuecomment-37568515) that would be nice "check to make sure the slug isn't set to null or an empty value", so I've made this in `needsSlugging` method. This way we can say "keep slug field unfilled to generate it automatically" even in new registers, in which a empty string is considered dirt enough.

Hope it is useful.