You need to create a formula with the following
shared numbervar SubAvgPerformed:=0
This formula needs to be placed in a section above the subreport in a section which would be run for each record i.e. not report header, page header - probably in Group Header 1 at a guess. Therefore everytime the subreport is run the variable should get reset. If a value is found in the subreport it will be displayed. If no value is found a 0 should be shown.
Chris