Fix query bug that prevented events with undefined end date being used.

web.py

@@ -94,8 +94,8 @@ def get_products(session):
 def get_event(session):
     return session.query(Event)\
                   .filter(Event.starts_at < datetime.now())\
-                  .filter((Event.ends_at > datetime.now()) |
-                          (Event.ends_at is None))\
+                  .filter((Event.ends_at.is_(None))
+                          | (Event.ends_at > datetime.now()))\
                   .one_or_none()