Allow non-string values for 'start_date' change tariff as API parameter

CHANGELOG:

### Fixed
- [#771](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/771) Allow non-string values for 'start_date' as change tariff API parameter 

Related card: https://trello.com/c/IdvbmDdf/1205-urgent-acceptar-data-de-canvi-de-tarifa-format-otrs

---

OTRS sends a '{}' value when date-formatted DF are empty, instead of an empty string

{
  "code": "56757",
  "product_code": "SE_SC_REC_BA_F_100_SF",
  "start_date": {}
}