Cannot read property 'final' of null

[jmatsushita]

I ran into this while trying to setup hypertrie as a kappa view dataDb. I think its related to me using the createHistoryStream in clearIndex https://github.com/kappa-db/kappa-view/blob/master/index.js#L34 instead of createKeyStream from a leveldb database. Might make more sense to use the createReadStream instead, but it triggered this error which I think can occur in other contexts (when there's batch deletions?)

.../node_modules/hypertrie/lib/history.js:47
    cb(null, node.final())
                  ^

TypeError: Cannot read property 'final' of null
    at done (.../node_modules/hypertrie/lib/history.js:47:19)
    at onnode (.../node_modules/hypertrie/index.js:335:56)
    at Request._callback (.../node_modules/hypercore/lib/storage.js:45:14)
    at Request.callback (.../node_modules/random-access-storage/index.js:173:8)
    at onread (.../node_modules/random-access-file/index.js:83:31)
    at FSReqCallback.wrapper [as oncomplete] (fs.js:561:5)

Given that nodeBySeq https://github.com/hypercore-protocol/hypertrie/blob/master/index.js#L332-L333 can return a null value, then createHistoryStream could also have a null returned as a node, and can't assume that it will have a .final() method.

PR with a fix coming up.