Fix issue for empty metadata in service directory resources (#15971)

Author: slevenick

mmv1/templates/terraform/encoders/service_directory_metadata_to_annotations.go.tmpl

@@ -1,5 +1,5 @@
 if obj["metadata"] == nil {
-	return nil, nil
+	return obj, nil
 }
 
 obj["annotations"] = obj["metadata"].(map[string]string)

mmv1/third_party/terraform/services/servicedirectory/resource_service_directory_endpoint_test.go.tmpl

@@ -91,11 +91,6 @@ resource "google_service_directory_endpoint" "example" {
   endpoint_id = "%s"
   service     = google_service_directory_service.example.id
 
-  metadata = {
-    stage  = "prod"
-    region = "us-central1"
-  }
-
   address = "1.2.3.4"
   port    = 5353
 }