[MJX] Don't raise sparse qM error in tendon_armature if there are no tendons.

PiperOrigin-RevId: 769753590
Change-Id: I5dd7a953f08e0566c5e22c2c926d7bd9c2c57bd6

Author: erikfrey

mjx/mujoco/mjx/_src/smooth.py

@@ -1191,6 +1191,9 @@ def tendon_armature(m: Model, d: Data) -> Data:
   if not isinstance(m._impl, ModelJAX) or not isinstance(d._impl, DataJAX):
     raise ValueError('tendon_armature requires JAX backend implementation.')
 
+  if not m.ntendon:
+    return d
+
   if not support.is_sparse(m):
     return d.tree_replace({
         '_impl.qM': (