Fix passing null as onFullfilled (which is broken in Chrome(-ium) 32)

Chromium v32 Promises implementation is rather old and supports only functions and ```undefined``` as ```onRejected``` and ```onFullfilled``` callbacks (otherwise it throws TypeError).

Some _local_ browsers are built on the top of Chromium 32 (e.g. http://s1.amigo.mail.ru/) and have not been updated yet. 

[JSFiddle](http://jsfiddle.net/palkan/4hptzyuw/) (Open in Chromium 32 to see a blank page)