[FIX] sale_crm: correctly handle customer option when converting opportunity

When converting an opportunity to a quotation, it will give 3 options where
the last option is "Do not link to a customer". When clicked it has the
same behavior than "Create a new customer".

Followup of 1d2bccd0db that added the
selection entry but did not implement its behavior.

This commit fixes that behavior. Now it opens a quotation without linked
customer.

Task ID 2065014

closes odoo/odoo#37026

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

addons/sale_crm/wizard/crm_opportunity_to_quotation.py

@@ -32,10 +32,11 @@ class Opportunity2Quotation(models.TransientModel):
             the freshly created opportunity view.
         """
         self.ensure_one()
-        self.lead_id.write({
-            'partner_id': self.partner_id.id if self.action == 'exist' else self._create_partner()
-        })
-        self.lead_id._onchange_partner_id()
+        if self.action != 'nothing':
+            self.lead_id.write({
+                'partner_id': self.partner_id.id if self.action == 'exist' else self._create_partner()
+            })
+            self.lead_id._onchange_partner_id()
         return self.lead_id.action_new_quotation()
 
     def _create_partner(self):