[FIX] link_tracker: do not die if statistic was unlinked when the user clicks on a tracked link
[FIX] link_tracker: do not die if statistic was unlinked when the user clicks on a tracked link

[There's a chance mail statistics disappear](https://github.com/odoo/odoo/blob/b3b8a471cc3e02488dcc0da991fd1d828978220d/addons/mass_mailing/models/mass_mailing.py#L519), which is probably not a very good decision, but there it is. Then, `add_click()` produces a 500 error when a user clicks on the mass mailing if the email comes from an old attempt.

Fix is to check the statistic record effectively exists before linking it.

Closes #22447.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo