Zepto#find(somethingFalsey) should return $()

Now, `Zepto#find(somethingFalsey)` return a pure empty array.
But I think `Zepto#find(somethingFalsey)` should return a `$()`.

Because:

 * We expect `Zepto#find` to return a collection

    In [the official documents](http://zeptojs.com/#find), we can not understand that "`Zepto#find` can return a pure array".

 * jQuery compatibility (for v1.11.1 and v2.1.1)

    I got `$()` by execute `$(elem).find(undefined)` on jQuery v1.11.1 and v2.1.1.