Adding empty tags to resource group

Adding empty tags to resource group to avoid message about external change in the challenge ("Test and Repair") that illustrates that Terraform is idempotent. User should see "No changes" without warning.

exercises/main.tf.start

@@ -14,6 +14,8 @@ provider "azurerm" {
 resource "azurerm_resource_group" "myresourcegroup" {
   name     = "${var.prefix}-workshop"
   location = var.location
+
+  tags = {}
 }
 
 # resource "azurerm_virtual_network" "vnet" {