Add assigned fix to mm fiber contract info

CHANGELOG:

### Added
- [#855](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/855) Add assigned fix to mm fiber contract info

Related card: https://trello.com/c/Rh4kHnW7/1316-afegir-camp-tel%C3%A8fon-fix-assignat-al-model-mmfiberservicecontractinfo

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