Add {exp:stash:is_empty} tag

Just a tiny little pull request for your consideration. I often find a need to test if a Stash variable IS empty, rather that if it is not. So I added an {exp:stash:is_empty} tag that is the exact negative of {exp:stash:not_empty}. Maybe there is a better way to handle this, but here's my code (all one line of it) in case you want it.