Do not expect receiving null from async.series

The async Node module can help write easier combinations of asynchronous JavaScript calls. When figuring out why a test kept on failing, it took me some time to find out that the err argument of the final callback of the series() function is undefined if no error occurred. This is different from the more common null value.

And series() isn't the only function that has this behavior. Keep it in mind when using async.

See this issue for more information.