Add check for left and right vn

left or right vn of a service chain could have been deleted while
schema transformer was down. When it comes back up, it tries to
read these networks. We should add a check for None before accessing
these networks to avoid generating a traceback.

Change-Id: If2d3ffccdc912eaa64a84e17233a2fb3e3a84477
Closes-Bug: 1590278
(cherry picked from commit d8518b1)

src/config/schema-transformer/config_db.py

@@ -1913,6 +1913,10 @@ def import_default_ri_route_target_to_service_ri(self):
             return
         left_vn = VirtualNetworkST.get(sc.left_vn)
         right_vn = VirtualNetworkST.get(sc.right_vn)
+        if left_vn is None or right_vn is None:
+            self._logger.debug("left or right vn not found for RI " + self.name)
+            return
+
         multi_policy_enabled = (
             left_vn.multi_policy_service_chains_enabled and
             right_vn.multi_policy_service_chains_enabled)