[fix] ERROR searx.engines.core.ac.uk: list index out of range

Some result items from core.ac.uk do not have an URL::

  Traceback (most recent call last):
  File "searx/search/processors/online.py", line 154, in search
    search_results = self._search_basic(query, params)
  File "searx/search/processors/online.py", line 142, in _search_basic
    return self.engine.response(response)
  File "SearXNG/searx/engines/core.py", line 73, in response
    'url': source['urls'][0].replace('http://', 'https://', 1),

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

searx/engines/core.py

@@ -53,6 +53,9 @@ def response(resp):
     for result in json_data['data']:
 
         source = result['_source']
+        if not source['urls']:
+            continue
+
         time = source['publishedDate'] or source['depositedDate']
         if time:
             date = datetime.fromtimestamp(time / 1000)