there are cases where we cannot classify a test example

When building features with a BOW/indexing approach, there might be a case where the example given to `classifier.predict` doesn't have any known features (e.g. a document with only words that have never been seen in the training set).

In such case, with scala, we can return an `Option` of the predicted label. If we can produce the prediction, we return `Some(label)`, if we can't make a prediction, we return `None`.