Summary error for no sample groups

ERROR:
Error in utils/summary_classes ProteinGroupSummary init if no sample groups are defined:
--> self.sample_group_to_sample_list_dict = None --> None has no len
self.len_quant = 4 * len(self.sample_group_to_sample_list_dict) + 4 * sum([len(sample_list) for sample_list in self.sample_group_to_sample_list_dict.values()])
SOLUTION:
default value for sample_groups = {}, instead of None (in create_summary.smk)