Fix sporadic CUDA launch failures with Cycles viewport denoising
Fix sporadic CUDA launch failures with Cycles viewport denoising

Sometimes the viewport buffer size is zero for a frame, which caused the denoising task to also try to
launch CUDA kernels with a launch size of zero, which in turn failed with a CUDA error. This patch
prevents launches from occuring in this case, similar to how it is handled in `copy_to_display_buffer`.

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird