Prevent negative item orders (#392)

* Pulling changes over from previous branch

* Adding exception and guard clause

src/ApplicationCore/Exceptions/EmptyBasketOnCheckoutException.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Exceptions
+{
+    public class EmptyBasketOnCheckoutException : Exception
+    {
+        public EmptyBasketOnCheckoutException()
+            : base($"Basket cannot have 0 items on checkout")
+        {
+        }
+
+        protected EmptyBasketOnCheckoutException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) : base(info, context)
+        {
+        }
+
+        public EmptyBasketOnCheckoutException(string message) : base(message)
+        {
+        }
+
+        public EmptyBasketOnCheckoutException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}

src/ApplicationCore/Exceptions/GuardExtensions.cs

@@ -1,5 +1,7 @@
 using Microsoft.eShopWeb.ApplicationCore.Exceptions;
 using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace Ardalis.GuardClauses
 {
@@ -10,5 +12,11 @@ namespace Ardalis.GuardClauses
             if (basket == null)
                 throw new BasketNotFoundException(basketId);
         }
+
+        public static void EmptyBasketOnCheckout(this IGuardClause guardClause, IReadOnlyCollection<BasketItem> basketItems)
+        {
+            if (!basketItems.Any())
+                throw new EmptyBasketOnCheckoutException();
+        }
     }
 }