[schema] Return None if cannot locate resource

When the schema locates a resource from database, it stores the
id/fq_name mapping in memory but in case the resource does not
exist, it tries to get id from a None value that results to
'AttributeError' exception.

Closes-bug: #1567267

Change-Id: If9387252b9da9093a939651fed8f7a5cbb09be04

src/config/schema-transformer/config_db.py

@@ -102,7 +102,7 @@ def reinit(cls):
     @classmethod
     def locate(cls, key, *args):
         obj = super(DBBaseST, cls).locate(key, *args)
-        if obj.obj.uuid not in cls._uuid_fq_name_map:
+        if obj and obj.obj.uuid not in cls._uuid_fq_name_map:
             cls._uuid_fq_name_map[obj.obj.uuid] = key
         return obj
     # end locate
@@ -370,7 +370,7 @@ def delete_inactive_service_chains(self, old_scs, new_scs=None):
 
         # Delete the service chains that are no longer active
         for remote_vn_name in old_scs:
-            # Get the Remote VNs in this VN's service chain and 
+            # Get the Remote VNs in this VN's service chain and
             # get a list of the remote service chains in the remote
             # VNs which has this VNs name.
             remote_vn = VirtualNetworkST.get(remote_vn_name)