Ok by default hana will show nulls as ? but it seems you have turned this setting off and are still seeing question marks. This tells me that it is not truly null. Plus seeing your screenshot shows a list of values including ?. If it were truly null it would say 'null' in the list of distinct values. So I think the column truly contains real question marks that were imported from your CSV. You could either 1) massage the CSV, remove all question marks and re-import or 2) perform an update on the data something like this;
UPDATE SCHEMA_NAME_HERE.SYSTEM_STATUS_TABLE SET T = '' WHERE T = '?'
Hope this helps!
-Patrick