Fix empty userName when running WebRazorPages by adding default username. (#80)

src/WebRazorPages/Constants.cs

@@ -4,5 +4,6 @@
     {
         public const string BASKET_COOKIENAME = "eShop";
         public const int ITEMS_PER_PAGE = 10;
+        public const string DEFAULT_USERNAME = "Guest";
     }
 }

src/WebRazorPages/ViewComponents/Basket.cs

@@ -38,7 +38,7 @@ namespace Microsoft.eShopWeb.RazorPages.ViewComponents
             {
                 return User.Identity.Name;
             }
-            return GetBasketIdFromCookie();
+            return GetBasketIdFromCookie() ?? Constants.DEFAULT_USERNAME;
         }
 
         private string GetBasketIdFromCookie()