win: assert non-empty $windows_sdk_version

BUG=chromium:1420723
Change-Id: I90c248bb7ad89b6def9782af41e5ec04a99adea7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122440
Commit-Queue: Marshall Greenblatt <marshall@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/tests/BUILD.gn

@@ -132,6 +132,7 @@ if (build_with_chromium && is_win &&
   assert(defined(windows_sdk_path))
   assert(defined(windows_sdk_version))
   assert(windows_sdk_path != "")
+  assert(windows_sdk_version != "")
 
   # Declare a copy target to copy DXC binary from Windows SDK for running Dawn
   # end-to-end test in Chromium.