[FIX] helpdesk_user_default_ticket_team: handle partner without linked user

``create`` accessed ``partner.user_ids[0]`` which raised ``IndexError``
when the ticket's partner had no linked user (e.g. a plain contact).

Use ``partner.user_ids[:1]`` so a partner without a user yields an empty
recordset, which the following ``if not user`` guard handles gracefully,
leaving the ticket's team untouched.

helpdesk_user_default_ticket_team/models/helpdesk_ticket.py

@@ -20,7 +20,8 @@ class HelpdeskTicket(models.Model):
                 partner = self.env["res.partner"].browse(vals.get("partner_id"))
                 if not partner:
                     continue
-                user = self.env["res.users"].browse(partner.user_ids[0].id)
+                # A partner may have no linked user (e.g. a plain contact)
+                user = partner.user_ids[:1]
                 if not user:
                     continue