[FIX] im_livechat: fix crash when user changes after starting livechat

Before this commit, a crash occurred when the user changed after
starting a livechat conversation.

Steps to reproduce (login => logout)
- Login on admin
- Go to the website
- Start a livechat conversation
- Logout
- Start typing on the chat window composer
- Crash due to `NotFound` raised after `notify_typing`

The same error could occur the other way around (logout => login).
This error is due to the user not being a channel member after it
changed.

In order to fix this issue, livechat session is cleared when the
user changes. Indeed, it the "new" user is not linked to the livechat
conversation so it makes no sense to display it.

fixes https://github.com/odoo/odoo/issues/128206


opw-3421386

closes odoo/odoo#128301

Signed-off-by: Matthieu Stockbauer (tsm) <tsm@odoo.com>