NULL Values in Metrics

Hi,

Hoping someone may be able to assist.

I'm trying to add multiple numeric fields in Explore,... previously in Insights I had as below (small sample, this contains over 50 metrics being summed).  

I have not been able to figure out how to replicate this Metric in Explore, the below works as long as there are no filters because there are no NULL values in the full ticketing history, but as soon as I apply a filter to a date range (e.g.  1 month) the Query returns blank.  I can't seem to get the correct syntax in the metric for dealing with the NULL values as a 0.

I tried using Result Manipulation > Metric Filter > Remove blank values but it didn't make any difference.

Thanks

Melanie