Use currency rounding when currency has no cash rounding set

Closes #14638
(grafted from c2e82186e7089618939ae9270490a68d2ca17622)

--HG--
branch : 7.0

modules/account_cash_rounding/currency.py

@@ -15,4 +15,8 @@ class Currency(metaclass=PoolMeta):
         digits=(12, Eval('digits', 6)))
 
     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)

modules/account_cash_rounding/tests/scenario_account_cash_rounding_alternate_currency.rst

@@ -98,6 +98,6 @@ Post invoice::
 
     >>> line_to_pay, = invoice.lines_to_pay
     >>> line_to_pay.debit, line_to_pay.credit
-    (Decimal('21.03'), Decimal('0'))
+    (Decimal('21.03'), Decimal('0.00'))
     >>> line_to_pay.amount_second_currency
     Decimal('42.05')