MInor update to damage_model.py

- Initialize the list in the nested dict when the scaling specs are parsed.

pelicun/model/damage_model.py

@@ -1405,6 +1405,8 @@ def parse_scaling_specification(scaling_specification: dict) -> dict:  # noqa: C
                         if fnumber is None:
                             msg = f'Invalid number in {css}: {number}'
                             raise ValueError(msg)
+                        if limit_state_id not in parsed_scaling_specification[key]:
+                            parsed_scaling_specification[key][limit_state_id] = []
                         parsed_scaling_specification[key][limit_state_id].append(
                             (capacity_adjustment_operation, fnumber)
                         )