fix(schema): Shared bond id is send as empty string from OTRS if null
CHANGELOG:
### Fixed
- [#1065](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1065): Remove "shared_bond_id" type restriction from cerverus schema
Related card: https://trello.com/c/PHdOJpxT/1694-falla-la-creaci%C3%B3-de-contractes-sense-id-dabonament-compartit
Edited by Gerard Funosas