SEBA-746 do not overwrite OLTDevice serial number with empty string;
Ensure OLTs with an incorrect serial number don't work

Change-Id: Idd1665ce427351195a8a848d8404aeeb4a997b98
diff --git a/xos/synchronizer/steps/test_sync_olt_device.py b/xos/synchronizer/steps/test_sync_olt_device.py
index 57330d2..40f13d8 100644
--- a/xos/synchronizer/steps/test_sync_olt_device.py
+++ b/xos/synchronizer/steps/test_sync_olt_device.py
@@ -88,6 +88,7 @@
         # feedback state
         o.device_id = None
         o.oper_status = None
+        o.serial_number= None
         o.of_id = None
         o.id = 1