async.each should not crash with undefined entry

[ajcrites]

var foo;
async.each(foo);

This will yield an error:

node_modules/async/lib/async.js:120
    if (!arr.length) {

I think that if arr is undefined it should be treated like an empty array or should trigger an error in the callback. I have a strong preference for the former, though.

[aearly]

Related: #667