Preprocessing does not process no data. Example, health check api (response code 200). It turns out that we use Javascript preprocessing to get the api status.
{if (value == '')

{        return 0; }

else 

{         return 1; }

}}

But when the api does not respond and we receive no data, preprocessing does not handle this, so there were no values. I think you need to add processing to preprocessing nodata.