Handle io.connect(null, opts) correctly

In migrating to Socket.IO 1.0.x, we had to change `null` into `"/"` in:

```
io.connect(null, opts)
```

( See https://github.com/audreyt/ethercalc/commit/adbc86e529e2edac99bbcc7e3063e84b773169e0#diff-d87f0347aff1316677a2d709ec4b4777R49 )

This is because `typeof null` is `object`, so the options are discarded.

Fixed by checking for truth value of `uri` before checking its type.