Fix manual subscription request vat message error
CHANGELOG:
### Fixed
- [#621](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/621) Fix manual subscription request vat message error
Related card: https://trello.com/c/x8WNPV4Z/949-missatge-error-conf%C3%BAs-sr-validation
Edited by Pelayo García