DASHBUILDE-223: fix NPE when exporting to Excel

[jhrcek]

I just extracted the "DataSet to SXSSFWorkbook" functionality to separate function for testability purposes (so that I don't have to deal with niogit FS etc) and changed the NPE causing line

cell.setCellValue(value.toString());

to

String val = value == null ? "" : value.toString();
cell.setCellValue(val);

Also used try-with-resources for output stream closing.

Reproduced the issue in showcase and after this fix applied it's working fine.

[dgutierr]

Merged.

Thanks! @jhrcek