[fix] engine piped: 'invalid content'

SearXNG does not allow a None value in the content field of a result item.

If the key (shortDescription, uploaderName) in the JSON response from piped
exists but is set to None, SearXNG ignores this result item::

  DEBUG   searx    : result: invalid content: { ..,  'content': None,  ..}

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