[FIX] base_import: cannot import midnight date

Steps to reproduce:
1. Install hr_attendance
2. Go to attendances
3. Select any line
4. Change the check-out date to next day
5. Change the check-out time to 00:00:00
6. Export that line, file format xlsx
7. Favorites > import records
8. Upload the exported file > Test
9. Column check_out contains incorrect values.
Error in line 2: unconverted data remains: 2023-06-11

Cause of the issue:
options.get('date_format') is empty

opw-3374883

closes odoo/odoo#133424

Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>