code: function create_meta_info (meta_title, meta_description, meta_sort) {
  
    var yamlDocument = {};
    var meta_info_is_present = meta_title || meta_description || meta_sort;
  
    if (meta_info_is_present) {
      if (meta_title)       { yamlDocument.Title       = meta_title;              }
      if (meta_description) { yamlDocument.Description = meta_description;        }
      if (meta_sort)        { yamlDocument.Sort        = parseInt(meta_sort, 10); }
  
      return '---\n' + yaml.safeDump(yamlDocument) + '---\n';
    } else {
      return '---\n---\n';
    }
  
  }
docs: Returns an empty string if all input strings are empty