Do not copy payment amount cache when copying account move line

Closes #15111
(grafted from 6f4a8415d5d522e028b06df7b406d63b73bf90d1)

--HG--
branch : 8.0

modules/account_payment/account.py

@@ -235,6 +235,7 @@ class MoveLine(metaclass=PoolMeta):
         else:
             default = default.copy()
         default.setdefault('payments', None)
+        default.setdefault('payment_amount_cache')
         return super().copy(lines, default=default)
 
     @classmethod