=BG= make slightly more performant

As suggested by @wgester https://github.com/Famous/famous/pull/290#issuecomment-55809359

However, instead of a short cut using `return null;` I think we should set `this._nextNode` and `this._previousNode` respectively, as these cached properties are accessed elsewhere. 

The performance difference should be negligible, since a minifier can optimise that into one statement, e.g.: `if(!len)return this._nextNode=null;` should it seem the optimisation necessary.

Returning null without setting `this._nextNode` and `this._previousNode` to `null` could lead to potential memory leaks, such as when `sequenceFrom()` is called with a **non-empty array**, and at least one render is performed, and subsequently, `sequenceFrom()` is called with an **empty array**, and at least one render is performed.



... come to think of it, someone should write a test case for that - this is just postulation on my part, have not actually tested it.