fix: fix clang detection on windows (#369)

Author: cloudhan

cuda/private/template_helper.bzl

@@ -166,7 +166,7 @@ def _generate_toolchain_clang_build(repository_ctx, cuda, clang_path_or_label):
     compiler_use_cc_toolchain_env = repository_ctx.os.environ.get("CUDA_COMPILER_USE_CC_TOOLCHAIN", "false")
     if compiler_use_cc_toolchain_env == "true":
         compiler_attr_line = "compiler_use_cc_toolchain = True,"
-    elif clang_path_or_label.startswith("//") or clang_path_or_label.startswith("@"):
+    elif clang_path_or_label != None and (clang_path_or_label.startswith("//") or clang_path_or_label.startswith("@")):
         # Use compiler_label
         compiler_attr_line = 'compiler_label = "%{{clang_label}}",'
         clang_label_for_subst = clang_path_or_label