Allow to create contracts without vodafone_contract_code
CHANGELOG:
### Changed
- [#979](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/979) Allow to create contracts without "vodafone_contract_code"
Related card: https://trello.com/c/Drus6Ddx/1452-modificaci%C3%B3-api-contractes
Edited by Gerard Funosas