Don't save an empty <default/> section.

PiperOrigin-RevId: 520594748
Change-Id: Ib5e1eb3a93e9d579adeda1052a2c29d724d6b497

test/xml/xml_native_writer_test.cc

@@ -52,6 +52,15 @@ using ::testing::NotNull;
 
 using XMLWriterTest = MujocoTest;
 
+TEST_F(XMLWriterTest, EmptyModel) {
+  static constexpr char xml[] = "<mujoco/>";
+  mjModel* model = LoadModelFromString(xml);
+  ASSERT_THAT(model, NotNull());
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, Not(HasSubstr("default")));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, SavesMemory) {
   {
     static constexpr char xml[] = R"(