You can do this by adding a line item to count the values you want to include, i.e.: add a line item 'Count' = If 'Value' <> 0 then 1 else 0. Then you can calculate the average with a formula that is 'Value' / 'Count'. You will need to change the summary type of this line item to either 'Formula' or 'Ratio'. If you choose ratio, you need to specify the Numerator / Denomator below, so you would select 'Value' / 'Count' in this case.
not entirely elegant but....you'll need to use a "signal row" = if number row <> 0 then 1 else 0….next add calc row that divides ‘number row’ by "signal row" (the count)…and in blueprint mode change the calc row from sum to ‘formula’.