[FIX] point_of_sale: avoid storing undefined in localstorage

addons/point_of_sale/static/src/js/db.js

@@ -479,7 +479,7 @@ var PosDB = core.Class.extend({
     },
     set_cashier: function(cashier) {
         // Always update if the user is the same as before
-        this.save('cashier', cashier);
+        this.save('cashier', cashier || null);
     },
     get_cashier: function() {
         return this.load('cashier');