Set previous_service default in BroadbandISPInfo portability
CHANGELOG:
### Changed
- [#672](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/672) Set previous_service default in BroadbandISPInfo portability
Related card: https://trello.com/c/8PmsW1Ec/1051-fix-camp-servei-previ-odoo
Edited by Pelayo García