Blender Git Loki

Fix fully transparent shadow catcher pass without catchers

Makes it so behavior of a shadow catcher pass is always predictable:
it is always possible to multiply it with a backdrop, regardless of
presence of shadow catcher object in the scene.

The downside it that this change makes it so extra memory is allocated
to store empty shadow catcher, and will make it so denoiser will an
extra work. Is possible to avoid, but it ends up in tricky checks, and
the situation is unlikely to be that common to justify making code
more complex.

Differential Revision: https://developer.blender.org/D11945