InformationWeightTransform hates empty columns

We seem to crash kernels when we pass a sparse matrix with an empty column to InformationWeight transformers fit transform.  

It comes up when we've got a fixed token dictionary but our training data is missing some of our vocabulary:
```
vect = NgramVectorizer(token_dictionary=token_dictionary).fit_transform(eventid_list)
vect1 = InformationWeightTransformer().fit_transform(vect)
```