Fix set None to ovn_lb in hm_sync

We try to fetch newest ovn_lb in hm_sync process, but
`_find_ovn_lb_from_hm_id` could give None as return ovn_lb value.
In that case, we better use existing ovn_lb that passing to hm_sync.

Change-Id: Ic856c4db76fb10808372ad64861b56a508bdcf8a

Author: ricolin

ovn_octavia_provider/helper.py

@@ -3750,7 +3750,8 @@ def hm_sync(self, hm, ovn_lb, pool_key):
         :param ovn_lb: The OVN LoadBalancer object that needs to be sync
         :param pool_key: The pool_key where health monitor is associated
         """
-        lbhcs, ovn_lb = self._find_ovn_lb_from_hm_id(hm[constants.ID])
+        lbhcs, _ovn_lb = self._find_ovn_lb_from_hm_id(hm[constants.ID])
+        ovn_lb = _ovn_lb if _ovn_lb else ovn_lb
         if not lbhcs:
             LOG.debug("Loadbalancer health check %s not found!",
                       hm[constants.ID])