Alerting: Fix terraform export of notification policy

When exporting a notification policy with no `group_by` the hcl exported is not valid as it is missing the required field.
This fixes the issue by initializing the `group_by` field with an empty list if it is nil.

pkg/services/ngalert/api/api_provisioning.go

@@ -614,6 +614,10 @@ func exportHcl(download bool, body definitions.AlertingFileExport) response.Resp
 
 		for idx, cp := range body.Policies {
 			policy := cp.RouteExport
+			if policy.GroupByStr == nil {
+				// required field, must be set to empty array
+				policy.GroupByStr = &[]string{}
+			}
 			resources = append(resources, hcl.Resource{
 				Type: "grafana_notification_policy",
 				Name: fmt.Sprintf("notification_policy_%d", idx+1),