[FIX] base: fix merge partner wizard

In the contacts app, you can select 2 partners and merge them.

Before this commit:

    - If a partner was added by a SQL query and not directly on Odoo,
      the create_date will be empty. This causes a crash because the
      merge wizard try to sort the partners by date

After this commit:

    - If a partner do not have a create_date, it will be considered
      as 01/01/1970. The merge wizard will sort the partners by IDs too.

OPW-2091925

closes odoo/odoo#39518

X-original-commit: deeb769c
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>