[FIX] crm: allow to unselect all predictive lead scoring settings

Now we can deselect all the optional fields to compute the predictive
lead scoring.

Task 2028666

closes odoo/odoo#34614

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/crm/models/res_config_settings.py

@@ -50,6 +50,8 @@ class ResConfigSettings(models.TransientModel):
         for setting in self:
             if setting.predictive_lead_scoring_fields:
                 setting.predictive_lead_scoring_fields_str = ','.join(setting.predictive_lead_scoring_fields.mapped('name'))
+            else:
+                setting.predictive_lead_scoring_fields_str = ''
 
     @api.depends('predictive_lead_scoring_start_date_str')
     def _compute_pls_start_date(self):