message: def_file_filter: Fix a bug when VC path is not found
  `find_vc_path(repository_ctx)` returns `None` if VC path is not found.
diff: @@ -29,7 +29,7 @@ def _def_file_filter_configure_impl(repository_ctx):
  repository_ctx.file("def_file_filter.py", "")
  return
  vc_path = find_vc_path(repository_ctx)
  - if vc_path == "visual-studio-not-found":
  + if vc_path == None:
  auto_configure_fail("Visual C++ build tools not found on your machine")
  undname = find_msvc_tool(repository_ctx, vc_path, "undname.exe")
  