VS: Fix CUDA CodeGeneration field for CMAKE_CUDA_ARCHITECTURES=OFF

If we add no CUDA architecture flags, write an empty `CodeGeneration`
field to tell the CUDA Toolkit's VS integration not to add its default
flags.  We already use this approach for `CMAKE_CUDA_ARCHITECTURES=all`
so that the `-arch=all` flag can be added via `AdditionalOptions`.

Fixes: #23490