LPS-48432 Shopping portlet throws an error with a minimum order amount a...

...nd a 0 quantity item at checkout

In checkout() method of cart.jsp, we use js(alert) to check the situation. So in order to match the check way, the fix also uses js to check before submit the form.

The issue was caused by item's count changed in UI. So we should check the issue by using real item's conut.
The code "ShoppingUtil.calculateActualPrice(item, 1)" means that we firstly get the one item's actualPrice.

thanks,
Hai