[FIX] l10n_co: crash when adding contact

Install l10n_co,contacts. Open contacts.

Several crash messages will popup because the override in l10n_co of
res_partner is not computing the 'l10n_co_verification_code' field for
every record. Changing the implementation to be sure to store some value
for every record fix the issue.

opw-2082583

closes odoo/odoo#38406

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>