Return empty value for payment fields when group has no payments

issue10470
review363391002

payment.py

@@ -148,7 +148,7 @@ class Group(ModelSQL, ModelView):
         payment = Payment.__table__()
 
         # initialize result and columns
-        result = defaultdict(defaultdict)
+        result = defaultdict(lambda: defaultdict(lambda: None))
         columns = [
             payment.group.as_('group_id'),
             Count(payment.group).as_('payment_count'),