Handle another case where creating a constant can fail (#2854)

Fixes #2847
diff --git a/source/opt/generate_webgpu_initializers_pass.cpp b/source/opt/generate_webgpu_initializers_pass.cpp
index 6438b21..eaed3c2 100644
--- a/source/opt/generate_webgpu_initializers_pass.cpp
+++ b/source/opt/generate_webgpu_initializers_pass.cpp
@@ -80,6 +80,8 @@
 
       changed = true;
       auto* constant_inst = GetNullConstantForVariable(inst);
+      if (!constant_inst) return Status::Failure;
+
       AddNullInitializerToVariable(constant_inst, inst);
     }
   }