[mod] return empty string on missing osm title

Sometimes openstreetmap returns with None as title. In these cases use an empty
string instead.

searx/engines/openstreetmap.py

@@ -43,7 +43,7 @@ def response(resp):
         if 'display_name' not in r:
             continue
 
-        title = r['display_name']
+        title = r['display_name'] or u''
         osm_type = r.get('osm_type', r.get('type'))
         url = result_base_url.format(osm_type=osm_type,
                                      osm_id=r['osm_id'])