Return undefined if values is undefined or null

I am trying to implement [traceur](https://github.com/google/traceur-compiler) as a hook into sails 0.11 (see https://github.com/balderdashy/sails/issues/2517) and the only issue, apart from hook order is the `Object.keys(values).forEach` line when `values === null`. 

This if and return statement fixed it for me and traceur/ES6/7 code is now working in sails.