code: def get_cgi_parameter_str_or_none(form: cgi.FieldStorage,
                                    key: str) -> Optional[str]:
      
      s = get_cgi_parameter_str(form, key)
      if s is None or len(s) == 0:
          return None
      return s
docs: Extracts a string parameter from a CGI form, or ``None`` if the key doesn't
      exist or the string is zero-length.