Fix/sponsor process with old member

CHANGELOG:

### Fixed
- [#1033](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1033) Fix process to sponsor an old member.

Related card: https://trello.com/c/eu6EKDMr/1621-validar-una-sr-canviant-la-relaci%C3%B3-amb-la-cooperativa

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