How to get my transaction search to return "0" instead of "no results found" if no events are found?

I am trying to use the transaction command to get duration between two events In case there are no such events, I would like the search to return 0 instead of "no results found". This following command isn't working: index=main host="xyz" | transaction startswith="keyword1" endswith="keyword2" | eval spent_time = duration | stats sum(spent_time) as total_spent_time | table total_spent_time | fillnull value=NULL