message: allow loop variables that are None
diff: @@ -533,7 +533,7 @@ def _loop_process_state(value: type, in_state: list, out_state: list,
  elif enoki.is_enoki_struct_v(t):
  for k, v in t.ENOKI_STRUCT.items():
  _loop_process_state(getattr(value, k), in_state, out_state, True)
  - elif hasattr(value, 'loop_put'):
  + elif hasattr(value, 'loop_put') or value is None:
  pass
  elif not in_struct:
  raise enoki.Exception(
  