[FIX] hr_expense: approval date

Before this PR from version 15 to 16.1, the approval date was not populated so
when you were in studio and add this field manually. The field stayed empty
which wasn't the behaviour we wanted.

When clicking the approve button, the approval date is now populated with the
complete date.

closes odoo/odoo#120165

Task-id: 3254147
X-original-commit: ef5715c3
Signed-off-by: Quentin De Paoli <qdp@odoo.com>
Signed-off-by: Maximilien La Barre (malb) <malb@odoo.com>