Rule: no-empty-class

The `no-empty-class` rule disallows the use of empty character classes in regular expression literals:

```js
var foo = /^abc[]/;
```

It does not disallow empty character classes in the `RegExp` constructor:

```js
var foo = new RegExp("^abc[]");
```

This rule matches the JSLint rule: http://jslinterrors.com/empty-class/