fix 0 tensor issue in matmul and scatter_nd (microsoft#25326)

seen with gemma3n embedding model

Author: guschmue

onnxruntime/core/providers/webgpu/math/matmul.cc

@@ -109,6 +109,10 @@ Status MatMul::ComputeInternal(ComputeContext& context) const {
 
   ORT_RETURN_IF_ERROR(helper.Compute(a->Shape(), b->Shape()));
   auto* output_tensor = context.Output(0, helper.OutputShape());
+  if (output_tensor->Shape().Size() == 0) {
+    // If the output tensor is empty, we can return early.
+    return Status::OK();
+  }
   bool has_bias = context.InputCount() > 2;
 
   if (helper.N() < 8 && helper.K() < 8) {  // call MatMulNaiveProgram

onnxruntime/core/providers/webgpu/tensor/scatter_nd.cc

@@ -153,6 +153,10 @@ Status ScatterND::ComputeInternal(ComputeContext& context) const {
   const size_t components = 1;
   auto output_size = static_cast<uint32_t>((indices_shape.SizeToDimension(indices_rank - 1) + components - 1) / components);
   auto* output = context.Output(0, input_shape);
+  if (output_size == 0) {
+    // If the output tensor is empty, we can return early.
+    return Status::OK();
+  }
   MLDataType data_type = input->DataType();
   const void* source = input->DataRaw();
   void* target = output->MutableDataRaw();