Fixes a random seed of zero being ignored

This was caused by zero being interpreted as false.
Hurray loose typing! Hurray Javascript! :D

You can see the problem behavior by loading up any exercise in sandcastle, e.g
http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/reading_pictographs_2.html?seed=0
and repeatedly reloading the page.

Discovered this from another reported issue, so zeroes as seeds *do* happen.