Prevent editing previous provider model to regular users

CHANGELOG:

### Changed
- [#800](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/800) Do not allow empty previous provider's name

Related card: https://trello.com/c/MqaQunGP/1263-error-de-formularis-en-la-c%C3%A0rrega-de-previous-providers

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Also, do not allow to empty a previous provider's name.