fix dropping of messages in handleMessage

```javascript
exports.handleMessage = function (hook, context, cb) {
  cb(null);
};
```

This should drop all messages, just like it is [documented](http://etherpad.org/doc/v1.4.0/#index_handlemessage). Currently, this does nothing, because all hook-results which are `==` to `undefined` are removed. Since `null` is `==` to `undefined`, the result is discarded and the code calling the `handleMessage` never gets the `null`, which should have caused it to drop the message.