[fix] engine duden - don't raise exception on empty result list

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

    httpx.HTTPStatusError: Client error '404 Not Found' for url\
      'https://www.duden.de/suchen/dudenonline/amazing'

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



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