Allow One Shot w/o cost products

CHANGELOG:

### FIXED
- [#738](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/738) Allow One Shot w/o cost products

Related card: https://trello.com/c/vEcYIbhl/1153-permetre-demanar-abonaments-addicionals-sense-cost-per-la-usu%C3%A0ria-a-odoo

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