[ntuple] Add check for default entry existence

Author: enirolf

tree/ntuple/v7/src/RNTupleModel.cxx

@@ -362,7 +362,8 @@ void ROOT::Experimental::RNTupleModel::RegisterSubfield(std::string_view qualifi
       parent = parent->GetParent();
    }
 
-   AddSubfield(qualifiedFieldName, *fDefaultEntry);
+   if (fDefaultEntry)
+      AddSubfield(qualifiedFieldName, *fDefaultEntry);
    fRegisteredSubfields.emplace(qualifiedFieldName);
 }
 

tree/ntuple/v7/test/ntuple_model.cxx

@@ -293,6 +293,19 @@ TEST(RNTupleModel, RegisterSubfield)
    }
 }
 
+TEST(RNTupleModel, RegisterSubfieldBare)
+{
+   auto model = RNTupleModel::CreateBare();
+   model->MakeField<CustomStruct>("struct");
+   model->RegisterSubfield("struct.a");
+   model->Freeze();
+
+   EXPECT_THROW(model->GetDefaultEntry(), RException);
+
+   const auto entry = model->CreateEntry();
+   EXPECT_TRUE(entry->GetPtr<float>("struct.a"));
+}
+
 TEST(RNTupleModel, CloneRegisteredSubfield)
 {
    auto model = RNTupleModel::Create();