[FIX] crm: set partner correctly while converting lead to opportunity

- Task: https://www.odoo.com/web/#id=10133&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.cPUeuuC5qdmBykVI
Why `!= 'nothing'` and not `== 'each_exist_or_create'` ? Does that mean that even if the partner already exists, we will recreate it?