Handle nil IAP in backend service decoder (#3459)

Signed-off-by: Modular Magician <magic-modules@google.com>

google/resource_compute_backend_service.go

@@ -1480,6 +1480,10 @@ func resourceComputeBackendServiceDecoder(d *schema.ResourceData, meta interface
 	// We need to pretend IAP isn't there if it's disabled for Terraform to maintain
 	// BC behaviour with the handwritten resource.
 	v, ok := res["iap"]
+	if !ok || v == nil {
+		delete(res, "iap")
+		return res, nil
+	}
 	m := v.(map[string]interface{})
 	if ok && m["enabled"] == false {
 		delete(res, "iap")