Sphere::Pdf does not return zero when wi points away from the sphere.

[GaryHuan9]

Thank you so much for the great book!

In chapter 14.2.2 Sampling Shapes, the second overload of Shape::Pdf returns 0 if wi does not intersect with the shape, which conceptually make sense. However, when Sphere::Pdf overrides this method, its implementation never returns 0 if the second (general sphere PDF) path is chosen, even if wi points away from the sphere. Is this a missed case in the code, or am I misunderstanding something here?