let read_res add structure even in non-resonance case

The method read_res sets structure[mat_id]['data'] for the first time, which is expected by other methods. If the LRP flag for the material is -1, there's no resonance data, and the method did not change anything.
So the small change added some lines to fill structure[mat_id]['data']  with a single entry for mat_id and empty values, even in case there is no resonance data to read.