Fix BadRequest error with parent_pack_contract_id
CHANGELOG:
### Fixed
- [#978](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/978) Fix BadRequest error with parent_pack_contract_id
Related card: https://trello.com/c/CDWrPQbV/1550-error-massiu-de-canvis-de-tarifa-en-el-queuejob-dodoo
Edited by Gerard Funosas