I'm not sure that Qlik behaved here really differently from Excel:
Update: screenshot is attached
What do you observed hasn't much to do with the loaded data (null, empty, missing) else with the fact that you create with your chart a new table - and in there context some values could become NULL. Within a tablebox they would be displayed as NULL respecitively the '-' as optical replace of it. But if you used an aggregation like sum() the result will be 0.
If you want to avoid it you will probably need to query for the result and then changing it, maybe with something like: