Add default or empty values for required resources referenced in CWL

def add_required_resources(resources):
    """Add default or empty values for required resources referenced in CWL
    """
    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