Avoid dividing by zero

CHANGELOG:

### FIXED
- [#599](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/599) Avoid dividing by zero

Related card: https://trello.com/c/tGWgsqRO/935-error-execuci%C3%B3-informe-deute-ven%C3%A7ut

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