Don't return empty string on parsing error.

whatformat/util/whatdateformat.py

@@ -25,7 +25,6 @@ def _get_all_format_strings(date_string, allow_reuse=False,
     try:
         parsed_datetime = parse(date_string)
     except ParserError:
-        yield ''
         return
     tokens = DATE_TOKEN_REGEX.findall(date_string)
     possible_matches = defaultdict(list)  # type: Dict[str, List[str]]