Use currency rounding when currency has no cash rounding set

Closes #14638
(grafted from c2e82186e7089618939ae9270490a68d2ca17622)

--HG--
branch : 6.0

modules/account_cash_rounding/currency.py

@@ -16,4 +16,8 @@ class Currency(metaclass=PoolMeta):
         depends=['digits'])
 
     def cash_round(self, amount, rounding=ROUND_HALF_EVEN):
-        return self._round(amount, self.cash_rounding, rounding)
+        if self.cash_rounding is not None:
+            factor = self.cash_rounding
+        else:
+            factor = self.rounding
+        return self._round(amount, factor, rounding)