Alerting: Fix receiver inheritance when provisioning a notification policy (#82007)

Terraform Issue: grafana/terraform-provider-grafana#1007
Nested routes should be allowed to inherit the contact point from the root (or direct parent) route but this fails in the provisioning API (it works in the UI)

pkg/services/ngalert/provisioning/notification_policies.go

@@ -70,6 +70,7 @@ func (nps *NotificationPolicyService) UpdatePolicyTree(ctx context.Context, orgI
 		return err
 	}
 
+	receivers[""] = struct{}{} // Allow empty receiver (inheriting from parent)
 	err = tree.ValidateReceivers(receivers)
 	if err != nil {
 		return fmt.Errorf("%w: %s", ErrValidation, err.Error())

pkg/services/ngalert/provisioning/notification_policies_test.go

@@ -92,6 +92,31 @@ func TestNotificationPolicyService(t *testing.T) {
 		require.Equal(t, "slack receiver", updated.Receiver)
 	})
 
+	t.Run("no root receiver will error", func(t *testing.T) {
+		sut := createNotificationPolicyServiceSut()
+
+		newRoute := createTestRoutingTree()
+		newRoute.Receiver = ""
+		newRoute.Routes = append(newRoute.Routes, &definitions.Route{
+			Receiver: "",
+		})
+
+		err := sut.UpdatePolicyTree(context.Background(), 1, newRoute, models.ProvenanceNone)
+		require.EqualError(t, err, "invalid object specification: root route must specify a default receiver")
+	})
+
+	t.Run("allow receiver inheritance", func(t *testing.T) {
+		sut := createNotificationPolicyServiceSut()
+
+		newRoute := createTestRoutingTree()
+		newRoute.Routes = append(newRoute.Routes, &definitions.Route{
+			Receiver: "",
+		})
+
+		err := sut.UpdatePolicyTree(context.Background(), 1, newRoute, models.ProvenanceNone)
+		require.NoError(t, err)
+	})
+
 	t.Run("not existing receiver reference will error", func(t *testing.T) {
 		sut := createNotificationPolicyServiceSut()