[CLI] Skip scripts which export null

When a script inside the test folder exports null, lab throws at line 91 of `lib/cli.js`
In my use case, and how I came across this error, I was trying to do something like this, which would set an environment variable if it wasn't already set:

```javascript
exports = module.exports = (function() {
    if (!process.env.LIMIT) {
        process.env.LIMIT = 200;
    }
})();
```