[fix] engine woxikon.de - don't raise exception on empty result list

Woxikon expects a word in German, so with query "foo" the site finds nothing and
respons a 404:

    httpx.HTTPStatusError: Client error '404 Not Found' \
      for url 'https://synonyme.woxikon.de/synonyme/foo.php'

[1] https://github.com/searxng/searxng/issues/1543#issuecomment-1193317054

Closes: https://github.com/searxng/searxng/issues/1543


Suggested-by: @allendema [1]
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>