Fix a crash in dm_control Python viewer during perturbations.

The action enum used to be an integer with the old Python bindings, but now it's an enum the NONE action evaluates as true.

PiperOrigin-RevId: 586990491
Change-Id: I0b366c2c87c9d6bae1f7c8c0aaeec619aa434704

Author: nimrod-gileadi

dm_control/viewer/renderer.py

@@ -254,7 +254,7 @@ def start_move(self, action, grab_pos):
 
   def tick_move(self, viewport_offset):
     """Transforms object's location/rotation by the specified amount."""
-    if self._action:
+    if self._action and self._action != mujoco.mjtMouse.mjMOUSE_NONE:
       mujoco.mjv_movePerturb(self._model.ptr, self._data.ptr, self._action,
                              viewport_offset[0], viewport_offset[1],
                              self._scene.ptr, self._perturb.ptr)