[FIX] mail: prevents crash when completing self repeating activities

Before this commit, completing an activity that was automatically
generating new activities was causing a crash as no action is returned
when they are completed.

This commit fixes this issue by handling the case where no action is
returned.

opw-2361674

closes odoo/odoo#60628

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>