According to the POI docs you have to use row.getLastCellNum() to iterate properly and take account of missing/blank cells:
https://poi.apache.org/spreadsheet/quick-guide.html#Iterator

However, this is one of the NotSupported operations of StreamingRow

Can you implement this, or alternatively, document how to use the existing implementation to handle blank cells.