handle NaN in sigma_detrend

[vincelhx]

@jean2262 proposes to add

    # prevent nan line
    valid_inc = next(
        (inc_angle.isel(line=i) for i, values in enumerate(inc_angle.line.values)
        if not np.isnan(inc_angle.isel(line=i).values).any()),
        None
    )

    if valid_inc is None:
        raise ValueError("No valid incidence angle lines found.")

here

xsarsea/src/xsarsea/detrend.py

Line 55 in aa60086

sigma0_gmf_sample = model(inc_angle.isel(line=0), wind_speed_gmf, wind_dir_gmf, broadcast=True)