Handle None in psm_list['rescoring_features']

[rodvrees]

At the moment, psm_utils' PepXMLReader reader passes by default:
rescoring_features = None

This gives the following error in MS2Rescore:

  File "/home/robbe/miniconda3/envs/rescore/lib/python3.10/site-packages/ms2rescore/__main__.py", line 237, in main
    rescore(configuration=config)
  File "/home/robbe/miniconda3/envs/rescore/lib/python3.10/site-packages/ms2rescore/core.py", line 55, in rescore
    psm_list_feature_names = {
  File "/home/robbe/miniconda3/envs/rescore/lib/python3.10/site-packages/ms2rescore/core.py", line 58, in <setcomp>
    for feature_name in psm_list_features.keys()
AttributeError: 'NoneType' object has no attribute 'keys'

None values should thus handle None values as empty lists for correct processing.