Check that element is a nullptr before casting.

The bug was caused by the fact that the `mjs_as*` functions would return a pointer to a `mjC*` object, even if the input `mjsElement` pointer was null. This would cause a segfault.

PiperOrigin-RevId: 651802736
Change-Id: Ibec00d269a4befadff8a68e0513fd381c7e38a2a

python/mujoco/specs_test.py

@@ -234,5 +234,21 @@ class SpecsTest(absltest.TestCase):
     self.assertEqual(spec.sensors[1].name, 'sensor2')
     self.assertEqual(spec.sensors[2].name, 'sensor3')
 
+  def test_iterators(self):
+    spec = mujoco.MjSpec()
+    geom1 = spec.worldbody.add_geom()
+    geom2 = spec.worldbody.add_geom()
+    geom3 = spec.worldbody.add_geom()
+    geom1.name = 'geom1'
+    geom2.name = 'geom2'
+    geom3.name = 'geom3'
+    geom = spec.worldbody.first_geom()
+    i = 1
+    while geom:
+      self.assertEqual(geom.name, 'geom' + str(i))
+      geom = spec.worldbody.next_geom(geom)
+      i += 1
+
+
 if __name__ == '__main__':
   absltest.main()