Guard against None PhysicsManager._cfg in _apply_sdf_config

The cloner calls _apply_sdf_config unconditionally on each prototype.
If PhysicsManager._cfg is None at that point, cfg.sdf_cfg would raise
AttributeError. Add an early return when cfg is None, matching the
guard already used in the cloner's pattern compilation code.

Author: vidurv-nvidia

source/isaaclab_newton/isaaclab_newton/physics/newton_manager.py

@@ -481,6 +481,8 @@ def _apply_sdf_config(cls, builder: ModelBuilder):
         from newton import GeoType, ShapeFlags
 
         cfg = PhysicsManager._cfg
+        if cfg is None:
+            return
         sdf_cfg = cfg.sdf_cfg
         if sdf_cfg is None:
             return