Make JavaScript String wrapper methods null/undefined-safe

A great benefit of Java String Utils libraries such as Apache Commons StringUtils is their null-safety. You should be able to provide null/undefined-safety to your JavaScript String wrapper functions, e.g.

```
expect(_s(null).toUpperCase().value()).to.be.a('null');
expect(_s(undefined).toUpperCase().value()).to.be.an('undefined');
```

I successfully spiked out this functionality with a couple of tweaks to **index.js**.

You may also want to add explicit null/undefined-safe wrapper functions, so that one can tap into this safety without chaining, e.g.

```
expect(_s.toUpperCase(null)).to.be.a('null');
expect(_s.toUpperCase(undefined)).to.be.an('undefined');
```

Start in index.js, where the issue reports that the functionality was prototyped, and inspect the existing chained and direct String wrapper methods. Confirm that null remains null and undefined remains undefined for toUpperCase-style calls, both through _s(value).method().value() and the explicit _s.method(value) form; the issue does not name a test file.

Vom Indexierungsmodell aus dem Issue-Text verfasst.