[FIX] point_of_sale: return raises an irrelevant warning

When returning an item by entering an orderline with a negative quantity
you got a warning message asking you to confirm a large payment
amount. Introduced by 5a3d9ce9ce.

Closes #13777

addons/point_of_sale/static/src/js/screens.js

@@ -1814,7 +1814,7 @@ openerp.point_of_sale.load_screens = function load_screens(instance, module){ //
             }
 
             // if the change is too large, it's probably an input error, make the user confirm.
-            if (!force_validation && (order.get_total_with_tax() * 1000 < order.get_total_paid())) {
+            if (!force_validation && order.get_total_with_tax() > 0 && (order.get_total_with_tax() * 1000 < order.get_total_paid())) {
                 this.gui.show_popup('confirm',{
                     title: _t('Please Confirm Large Amount'),
                     body:  _t('Are you sure that the customer wants to  pay') +