migrator: interfaces: T4171: bugfix ConfigTreeError

Migrating 1.2.8 -> 1.4-rolling-202201110811

vyos-router[970]: Waiting for NICs to settle down: settled in 0sec..
vyos-router[1085]: Started watchfrr.
vyos-router[970]: Mounting VyOS Config...done.
vyos-router[970]: Starting VyOS router: migrate
vyos-router[1490]: Traceback (most recent call last):
vyos-router[1490]:   File "/opt/vyatta/etc/config-migrate/migrate/interfaces/5-to-6", line 112, in <module>
vyos-router[1490]:     for if_type in config.list_nodes(['interfaces']):
vyos-router[1490]:   File "/usr/lib/python3/dist-packages/vyos/configtree.py", line 236, in list_nodes
vyos-router[1490]:     raise ConfigTreeError("Path [{}] doesn't exist".format(path_str))
vyos-router[1490]: vyos.configtree.ConfigTreeError: Path [b'interfaces'] doesn't exist
vyos-router[1455]: Migration script error: /opt/vyatta/etc/config-migrate/migrate/interfaces/5-to-6: Command
     '['/opt/vyatta/etc/config-migrate/migrate/interfaces/5-to-6', '/opt/vyatta/etc/config/config.boot']'
     returned non-zero exit status 1..
vyos-router[970]:  configure.
vyos-config[979]: Configuration success

1 files changed, 7 insertions, 2 deletions

diff --git a/src/migration-scripts/interfaces/5-to-6 b/src/migration-scripts/interfaces/5-to-6
index ae79c1d1b..dc8d9554f 100755
--- a/src/migration-scripts/interfaces/5-to-6
+++ b/src/migration-scripts/interfaces/5-to-6
@@ -107,10 +107,15 @@ if __name__ == '__main__':
         config_file = f.read()
 
     config = ConfigTree(config_file)
+    base = ['interfaces']
+
+    if not config.exists(base):
+        # Nothing to do
+        exit(0)
 
     # list all individual interface types like dummy, ethernet and so on
-    for if_type in config.list_nodes(['interfaces']):
-        base_if_type = ['interfaces', if_type]
+    for if_type in config.list_nodes(base):
+        base_if_type = base + [if_type]
 
         # for every individual interface we need to check if there is an
         # ipv6 ra configured ... and also for every VIF (VLAN) interface