Fix handling country=None in attendee profiles (#6309)

* Fix handling country=None in AttendeeProfile describe

* Update checkoutflow.py

* fix more occurences

* handle country=None in InvoiceAddress even if it is not allowed

src/pretix/base/models/orders.py

@@ -1697,7 +1697,7 @@ class AbstractPosition(RoundingCorrectionMixin, models.Model):
             self.company,
             self.street,
             (self.zipcode or '') + ' ' + (self.city or '') + ' ' + (self.state_for_address or ''),
-            self.country.name
+            self.country.name if self.country else ''
         ]
         lines = [r.strip() for r in lines if r]
         return '\n'.join(lines).strip()
@@ -3438,7 +3438,7 @@ class InvoiceAddress(models.Model):
             self.name,
             self.street,
             (self.zipcode or '') + ' ' + (self.city or '') + ' ' + (self.state_for_address or ''),
-            self.country.name,
+            self.country.name if self.country else '',
             self.vat_id,
             self.custom_field,
             self.internal_reference,