Fix creation of regtypes that have cost

Commit f128188 back in 2018 made it impossible to change the cost of a
registration type if it had been used. This apparently also broke
creation of new registration types that had a cost from the first save,
which apparently hasn't really been used since. (It would not break
copy-from-other-conference, so maybe that's what people used)

postgresqleu/confreg/backendforms.py

@@ -473,7 +473,7 @@ def clean_cost(self):
         if self.cleaned_data['cost'] > 0 and not self.conference.paymentmethods.exists():
             raise ValidationError("Cannot assign a cost, this conference has no payment methods")
 
-        if self.instance and self.instance.cost != self.cleaned_data['cost']:
+        if self.instance and self.instance.pk and self.instance.cost != self.cleaned_data['cost']:
             if self.instance.conferenceregistration_set.filter(Q(payconfirmedat__isnull=False) | Q(invoice__isnull=False) | Q(bulkpayment__isnull=False)).exists():
                 raise ValidationError("This registration type has been used, so the cost can no longer be changed")