code: def add_required_resources(resources):
      
      required = [["variation", "cosmic"], ["variation", "clinvar"], ["variation", "dbsnp"],
                  ["variation", "lcr"], ["variation", "polyx"],
                  ["variation", "encode_blacklist"], ["variation", "gc_profile"],
                  ["variation", "germline_het_pon"],
                  ["variation", "train_hapmap"], ["variation", "train_indels"],
                  ["variation", "editing"], ["variation", "exac"], ["variation", "esp"],
                  ["variation", "gnomad_exome"],
                  ["variation", "1000g"], ["aliases", "human"]]
      for key in required:
          if not tz.get_in(key, resources):
              resources = tz.update_in(resources, key, lambda x: None)
      return resources
docs: Add default or empty values for required resources referenced in CWL