model-loader : skip TENSOR_SKIP tensors in the metadata-only path

create_tensor asserts on a null buffer type when building from metadata
alone, but buft_for_tensor returns null by design for tensors marked
TENSOR_SKIP, which is how architectures with nextn/MTP layers mark theirs.
Those models cannot be constructed by llama_model_init_from_user at all.

The file-backed path below already returns nullptr for the same tensors, so
callers see the same thing either way.

src/llama-model-loader.cpp

@@ -1225,7 +1225,7 @@ struct ggml_tensor * llama_model_loader::create_tensor(
     };
 
     if (files.empty()) {
-        if (flags & TENSOR_SKIP_IF_VIRTUAL) {
+        if (flags & (TENSOR_SKIP_IF_VIRTUAL | TENSOR_SKIP)) {
             return nullptr;
         }
         ggml_type type = GGML_TYPE_F32;