Bug: The redirection feature for a merged doc return a bad url

[fbunoz]

Describe the bug
An access to the url of a merged doc is redirected to a bad url.
All redirected url have this format : https://www.camptocamp.org/articles//undefined/

To Reproduce
Try this URL : https://www.camptocamp.org/articles/106746
It should be redirected to https://www.camptocamp.org/articles/164306, but it is redirected to https://www.camptocamp.org/articles/164306/undefined/

Same problem with https://www.camptocamp.org/articles/106746/fr or https://www.camptocamp.org/articles/106746/fr/titre