Fixing GetCart for non-existing user

Now GetCart returns an empty cart for non-existing user
(user that haven't added anything to the cart before)
Added test to cover that

Also enhanced windows script for running cart service locally.
Now we have two options:
 - Running the service and redis locally (assuming redis emulator is installed)
   This is good for easier local debugging and troubleshooting

   docker_setup local

 - Running the service and redis in two separate docker containers
   This is good for better simulation of what will happen in the cloud

tests/cartservice/CartServiceTests.cs

@@ -12,6 +12,29 @@ namespace cartservice
         private static string serverHostName = "localhost";
         private static int port = 7070;
 
+        [Fact]
+        public async Task GetItem_NoAddItemBefore_EmptyCartReturned()
+        {
+            string userId = Guid.NewGuid().ToString();
+
+            // Construct server's Uri
+            string targetUri = $"{serverHostName}:{port}";
+
+            // Create a GRPC communication channel between the client and the server
+            var channel = new Channel(targetUri, ChannelCredentials.Insecure);
+
+            var client = new CartServiceClient(channel);
+
+            var request = new GetCartRequest
+            {
+                UserId = userId,
+            };
+            var cart = await client.GetCartAsync(request);
+            Assert.NotNull(cart);
+            // All grpc objects implement IEquitable, so we can compare equality with by-value semantics
+            Assert.Equal(new Cart(), cart);
+        }
+
         [Fact]
         public async Task AddItem_ItemInserted()
         {