[fix] unsplash engine - 'searx:result: invalid title:'

- Use result 'alt_description' as title, if not given use
  default title 'unknown'.
- Use result 'description' from unsplash as 'content'

Fix error::

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

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

searx/engines/unsplash.py

@@ -62,7 +62,8 @@ def response(resp):
                 'url': clean_url(result['links']['html']),
                 'thumbnail_src': clean_url(result['urls']['thumb']),
                 'img_src': clean_url(result['urls']['raw']),
-                'title': result['description'],
-                'content': ''
+                'title': result.get('alt_description') or 'unknown',
+                'content': result.get('description') or ''
             })
+
     return results