Fix fields for PurchaseOrderCancelSerializer (#7481)

- Throwing an error on an OPTIONS request

src/backend/InvenTree/order/serializers.py

@@ -270,7 +270,7 @@ class PurchaseOrderCancelSerializer(serializers.Serializer):
     class Meta:
         """Metaclass options."""
 
-        fields = ([],)
+        fields = []
 
     def get_context_data(self):
         """Return custom context information about the order."""

src/backend/InvenTree/order/test_api.py

@@ -484,6 +484,9 @@ class PurchaseOrderTest(OrderTest):
 
         url = reverse('api-po-cancel', kwargs={'pk': po.pk})
 
+        # Get an OPTIONS request from the endpoint
+        self.options(url, data={'context': True}, expected_code=200)
+
         # Try to cancel the PO, but without required permissions
         self.post(url, {}, expected_code=403)