Issues in calculations in pie chart panel

I am using prometheus as Data source. I have two queries one gets the metrics in GB and other one gets the metrics in the Bytes. I’m trying to add both values after converting the Bytes one into GB.
But there is this case when one of the query does not fetch any metrics or it is null, It is showing the No data points, it is not even showing the metrics for other query which is fetching the metrics. Is there any way to add null and show results ?
Also I tried creating the separate variables and using those variables to do the same but the problem persists.
will this solve problem if get both metrics fetched in GB and we just have to do the sum without any conversion??
any pointers will be appreciated . Thanks