metadata file never loaded if not provided

In this line detect_shaded_snow is always False (https://gitlab.orfeo-toolbox.org/remote_modules/let-it-snow/-/blob/int_stage_roman/python/s2snow/fsc_config.py#L506)

if self.detect_shaded_snow and (self.relief_shadow_mask is None):

Since it is initialized to False L86..