Let camera parameter "focusDistance" default to 1

Note that this is the default specified by ANARI: https://registry.khronos.org/ANARI/specs/1.0/ANARI-1.0.html#object_types_camera

Some ray tracing-based ANARI backends that implement thin-lens camera models assume the distance to be set and then later divide by zero when generating primary rays. Though this is something the backend could (and probably should check), it would increase overall robustness if VTK followed the specs here.