[fix] qwant engine - prevent exception on date/time value is None

Has been reported in [1], error messages::

  Error
       Error: ValueError
       Percentage: 0
       Parameters: ()
       File name: searx/engines/qwant.py:159
       Function: response
       Code: pub_date = datetime.fromtimestamp(item['date'], None)

    Error
        Error: TypeError
        Percentage: 0
        Parameters: ('an integer is required (got type NoneType)',)
        File name: searx/engines/qwant.py:196
        Function: response
       Code: pub_date = datetime.fromtimestamp(item['date'])

Fix timedelta from seconds to milliseconds [1], error message::

    Error
        Error: TypeError
        Percentage: 0
        Parameters: ('unsupported type for timedelta seconds component: NoneType',)
        File name: searx/engines/qwant.py:195
        Function: response
        Code: length = timedelta(seconds=item['duration'])

[1] https://github.com/searxng/searxng/issues/222



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>