FindCUDAToolkit: Handle toolkits that don't provide nvptxcompiler

Some versions of the CUDA toolkit don't provide any static
libraries, and therefore we need to handle that `nvptxcompiler_static`
might not be found

Modules/FindCUDAToolkit.cmake

@@ -1060,7 +1060,9 @@ if(CUDAToolkit_FOUND)
   if(CUDAToolkit_VERSION VERSION_GREATER_EQUAL 11.1.0)
     if(NOT TARGET CUDA::nvptxcompiler_static)
       _CUDAToolkit_find_and_add_import_lib(nvptxcompiler_static DEPS cuda_driver)
-      target_link_libraries(CUDA::nvptxcompiler_static INTERFACE Threads::Threads)
+      if(TARGET CUDA::nvptxcompiler_static)
+        target_link_libraries(CUDA::nvptxcompiler_static INTERFACE Threads::Threads)
+      endif()
     endif()
   endif()