Order creation API: Do not allow empty orders

src/pretix/api/serializers/order.py

@@ -557,7 +557,7 @@ class CompatibleJSONField(serializers.JSONField):
 
 class OrderCreateSerializer(I18nAwareModelSerializer):
     invoice_address = InvoiceAddressSerializer(required=False)
-    positions = OrderPositionCreateSerializer(many=True, required=False)
+    positions = OrderPositionCreateSerializer(many=True, required=True)
     fees = OrderFeeCreateSerializer(many=True, required=False)
     status = serializers.ChoiceField(choices=(
         ('n', Order.STATUS_PENDING),

src/tests/api/test_orders.py

@@ -159,6 +159,7 @@ TEST_PAYMENTS_RES = [
         "created": "2017-12-01T10:00:00Z",
         "payment_date": "2017-12-01T10:00:00Z",
         "provider": "stripe",
+        "payment_url": None,
         "state": "refunded",
         "amount": "23.00"
     },
@@ -167,6 +168,7 @@ TEST_PAYMENTS_RES = [
         "created": "2017-12-01T10:00:00Z",
         "payment_date": None,
         "provider": "banktransfer",
+        "payment_url": None,
         "state": "pending",
         "amount": "23.00"
     }