[FIX] partner_event: NewId compatibility

When the compute is triggered from a NewId the domain will fail when
it's evaluated as expression parser can't deal with it.

TT45858

partner_event/models/res_partner.py

@@ -27,7 +27,7 @@ def _compute_registration_count(self):
                 self.env["event.registration"]
                 .search(
                     [
-                        ("attendee_partner_id", "child_of", partner.id),
+                        ("attendee_partner_id", "child_of", partner.ids),
                         ("state", "not in", ("cancel", "draft")),
                     ]
                 )