Don't cache the vlan-id if it is not valid from DB (#273)

Don't cache the vlan-id if it is not valid from DB

**- What I did**
Avoid caching the vlan-id with invalid value.

**- How I did it**
Add a check in code so if vlan-id is not valid, don't cache it.

src/sonic_ax_impl/mibs/ietf/rfc4363.py

@@ -31,7 +31,9 @@ def fdb_vlanmac(self, fdb):
                 vlan_id = Namespace.dbs_get_vlan_id_from_bvid(self.db_conn, fdb["bvid"])
                 if isinstance(vlan_id, bytes):
                     vlan_id = vlan_id.decode()
-                self.bvid_vlan_map[fdb["bvid"]] = vlan_id
+                # only cache vlan_id if valid
+                if vlan_id is not None:
+                    self.bvid_vlan_map[fdb["bvid"]] = vlan_id
         else:
             return None
         if not isinstance(vlan_id, str):