When a new user is subscribed to a channel, (multi-user, without emails)
a message is posted on this channel.
The mechanic used for that is unfortunately not very reliable
Javascript will call channel_join_and_get_info every time a channel
is added (client side) even if the user is already in the channel.
This fix simply check that the action_follow actually add the
user to the channel before sending the message.
We should avoid to call channel_join_and_get_info in this case and only call
channel_info but this fix will temporary fix the issue.
closesodoo/odoo#29441