ExternalTextureTransform: Fix undefined behavior

WGSL now supports type inferencing with `let` declarations, so `var->type()` may return nullptr.
Calling a method (`Is<T>()`) on a nullptr object is UB. Instead used the free-function form which can handle nullptr objects.

Change-Id: I8faf875e1550eac1306ad731a65f3662bc4e5b43
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50307
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Jones <brandon1.jones@intel.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/transform/external_texture_transform.cc

diff --git a/src/transform/external_texture_transform.cc b/src/transform/external_texture_transform.cc
index eade354..e3c61ff 100644
--- a/src/transform/external_texture_transform.cc
+++ b/src/transform/external_texture_transform.cc
@@ -28,7 +28,7 @@
   // Scan the AST nodes for external texture declarations.
   for (auto* node : ctx.src->ASTNodes().Objects()) {
     if (auto* var = node->As<ast::Variable>()) {
-      if (var->type()->Is<ast::ExternalTexture>()) {
+      if (Is<ast::ExternalTexture>(var->type())) {
         // Replace a single-plane external texture with a 2D, f32 sampled
         // texture.
         auto newType = ctx.dst->ty.sampled_texture(ast::TextureDimension::k2d,