[Bug] Seemingly unsupported Grid2Op v1.13 feature

[alrazol]

We get the following error trying to init a Grid2Op env with PyPowSyBl2Grid backend with Grid2Op v1.13 and not with earlier versions of it. The error is:

Exception has occurred: TypeError       (note: full exception trace is shown but execution is paused at: _run_module_as_main)
'NoneType' object is not subscriptable
  File "/home/razolalf/Documents/Code/pf_projector/.venv/lib/python3.12/site-packages/grid2op/Backend/backend.py", line 2245, in get_shunt_setpoint
    p_s[sh_conn] *= (self._sh_vnkv[sh_conn] / sh_v[sh_conn]) ** 2
                     ~~~~~~~~~~~~~^^^^^^^^^
  File "/home/razolalf/Documents/Code/pf_projector/.venv/lib/python3.12/site-packages/grid2op/Environment/_env_prev_state.py", line 172, in update_from_backend
    shunt_p, shunt_q, shunt_bus = backend.get_shunt_setpoint()

This seems to be linked to changes made in Grid2Op v1.13 associated with changes made in this PR

To reproduce : Try to init a grid2Op env with this backend and Grid2Op v1.13:

backend = PyPowSyBlBackend(
        detailed_infos_for_cascading_failures=False,
        can_be_copied=True,
        check_isolated_and_disconnected_injections=None,
        consider_open_branch_reactive_flow=False,
        connect_all_elements_to_first_bus=False,
        n_busbar_per_sub=6,
    )
backend._prevent_automatic_disconnection = False

Then

grid2op_env = grid2op.make(
        grid2op_env_dir,
        backend=backend,
        allow_detachment=True,  # True because in the initial grid.xiidm some loads are disconnected so we need to drop them
        n_busbar=6,
        chronics_class=ChangeNothing,
        param=grid2op_params,
    )

With an .xiidm network simply loaded from PF_STANWAY/2026/01/22/PtFige-20260122-1055-enrichi.arc.xz