VOL-4605 ONU Adapter return nil when getting ONU Indication Down. ONU adapter declared as failed for this issue then It was success.

Change-Id: I2b4cf3cda2dbdc54d1ef41e109185388ff568117
diff --git a/internal/pkg/core/openonu.go b/internal/pkg/core/openonu.go
index c5edad8..f9e253e 100755
--- a/internal/pkg/core/openonu.go
+++ b/internal/pkg/core/openonu.go
@@ -896,7 +896,10 @@
 			}
 			return &empty.Empty{}, nil
 		} else if (onuOperstate == "down") || (onuOperstate == "unreachable") {
-			return nil, handler.updateInterface(ctx, onuIndication)
+			if err := handler.updateInterface(ctx, onuIndication); err != nil {
+				return nil, err
+			}
+			return &empty.Empty{}, nil
 		} else {
 			logger.Errorw(ctx, "unknown-onu-ind-request operState", log.Fields{"OnuId": onuIndication.GetOnuId()})
 			return nil, fmt.Errorf("invalidOperState: %s, %s", onuOperstate, onuInd.DeviceId)