check for NULL when parsing strings

to reproduce the issue:

```
$ build/duklub repl.js
```

```js
fd = uv.fs_open('./b.js', 'r', 0644);
uv.fs_read(fd, 1024, 0).toString();
```

before the change, an error trying to parse an empty string (NULL), after the change, expected behavior.
I see this was a typo on my part.  I'd rather change this to simply `while (string[0]) {` if you don't mind.  This function assumes string is not NULL since it's always coming from js via `duk_require_string`.