[FIX] mass_mailing: wrong clicks_ratio compute

The `clicks_ratio` field was computed outside of its compute method,
this is illegale as the ORM cannot protect the field from being included
in the "towrite" structure.

Backport of 7acc035

closes odoo/odoo#67097

X-original-commit: ca1f2e9d
Signed-off-by: Julien Castiaux <Julien00859@users.noreply.github.com>