count(), unique_count() and sum() should default to 0 when there’s no data (null).
For other functions there’s the default() which does when @carly.richmond is achieving with ifelse.
Unfortunately defaults() won’t work where there are no records at all, not sure it’s your case, but there’s an issue you can track about it here: [Kibana Lens] Possibility to display 0 or any other default value instead '-' in a cell when there's no record to display · Issue #205085 · elastic/kibana · GitHub