[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>