Don't save actdim="{0,1}" for state{less,ful} actuators, respectively.

actdim has a default value of -1 (==unset), which means it currently gets saved for all actuators. After this change, it will only be saved if the value is > 1, errors are thrown for stateful actuators with actdim=0 and stateless actuators with actdim>0.

PiperOrigin-RevId: 520627313
Change-Id: Ia498dae5c7314cfaa0995f8077d297c2339a5c35

test/xml/xml_native_writer_test.cc

@@ -844,6 +844,8 @@ TEST_F(XMLWriterTest, Actdim) {
     </worldbody>
     <actuator>
       <general joint="hinge" dyntype="user" actdim="2"/>
+      <general joint="hinge" dyntype="filter" dynprm="1"/>
+      <motor joint="hinge"/>
     </actuator>
   </mujoco>
   )";
@@ -851,6 +853,8 @@ TEST_F(XMLWriterTest, Actdim) {
   ASSERT_THAT(model, NotNull());
   std::string saved_xml = SaveAndReadXml(model);
   EXPECT_THAT(saved_xml, HasSubstr("actdim=\"2\""));
+  EXPECT_THAT(saved_xml, Not(HasSubstr("actdim=\"1\"")));
+  EXPECT_THAT(saved_xml, Not(HasSubstr("actdim=\"0\"")));
   mj_deleteModel(model);
 }