Fix activity_type for One Shot w/o Cost

CHANGELOG:

### FIXED
- [#840](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/840) Fix activity_type for One Shot w/o Cost

Related card: https://trello.com/c/J8uzLAH3/1312-wizard-oneshot-crea-activitat-canvi-sim

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