Raise an error if MJX scan functions are called on models with zero DoFs.

In the future, MJX will support correctly simulating models with zero dofs.

PiperOrigin-RevId: 760731314
Change-Id: Iadddf80a959c64cddb31610e60927ae0437ebcaf

mjx/mujoco/mjx/_src/scan.py

@@ -132,6 +132,8 @@ def _nvmap(f: Callable[..., Y], *args) -> Y:
 
 def _check_input(m: Model, args: Any, in_types: str) -> None:
   """Checks that scan input has the right shape."""
+  if m.nv == 0:
+    raise ValueError('Scan across Model with zero DoFs unsupported.')
   size = {
       'b': m.nbody,
       'j': m.njnt,

mjx/mujoco/mjx/_src/scan_test.py

@@ -61,10 +61,7 @@ class ScanTest(absltest.TestCase):
       return body_id + 1
 
     b_in = jp.array([1])
-    b_expect = jp.array([2])
-    b_out = scan.flat(m, fn, 'b', 'b', b_in)
-
-    np.testing.assert_equal(np.array(b_out), np.array(b_expect))
+    self.assertRaises(ValueError, scan.flat, m, fn, 'b', 'b', b_in)
 
   def test_flat_joints(self):
     """Tests scanning over bodies with joints of different types."""