tfidf should return empty map instead of nothing

For this call:

```clojure
(tf-idf ["" "hello"])
```

I think it should be better to return
```clojure
( {}  {"hello" -0.6931471805599453})
```

but I get 

```clojure
({"hello" -0.6931471805599453})
```

If I give it x documents (whatever content they have),
I would expect to get back a list of x tfidf-maps, always. 
