Output blank cells instead of cells containing "null"

view/src/main/java/se/su/dsv/scipro/io/ExcelExporter.java

@@ -72,7 +72,9 @@ public class ExcelExporter extends AbstractDataExporter {
         for (int i = 0; i < columns.size(); i++) {
             Object cellValue = columns.get(i).getDataModel(data).getObject();
             Cell cell = row.createCell(i);
-            cell.setCellValue(String.valueOf(cellValue));
+            if (cellValue != null) {
+                cell.setCellValue(cellValue.toString());
+            }
         }
     }
 }