Updated _F.js to allow _F(0)

Hey there!  Thanks for starting this project.  I've just gotten started with d3.js and I'm finding this library super helpful.

An issue I found is, when using d3.zip you end up with an array of arrays instead of an array of objects.  I want to be able to use _F(0) and _F(1) to access the relevant values.

_F(1) is fine, but _F(0) trips up when the argument is checked for falsiness.

I'm not an expert on truthiness in javascript, so I just changed the truthiness check to explicitly allow 0 values.