Set a default value when calculating clearing reconcile of a payment group

Closes #14301
(grafted from 91174ccc69295c1735dad93d720e078ceff91e51)

--HG--
branch : 7.0

modules/account_payment_clearing/payment.py

@@ -395,7 +395,7 @@ class Group(metaclass=PoolMeta):
         Payment = pool.get('account.payment')
         payment = Payment.__table__()
         cursor = Transaction().connection.cursor()
-        result = defaultdict()
+        result = defaultdict(lambda: None)
         column = Coalesce(payment.clearing_reconciled, False)
         if backend.name == 'sqlite':
             column = Min(column)