return null for error in callback

[schloerke]

The paradigm for node.js callbacks is to return a 'null' object if no errors are found for the error value.

This allows for quick

if (responseError) fix_it();

Best,
Barret

[3rd-Eden]

There are probably a few more cases where i'm not returning null but false so beware of that as well if you are going to strictly check for null values.

But thanks for the pull request!