I guess that forcing cell type to HSSFCell.CELL_TYPE_STRING might be the reason of getting null.
A more robust way to obtain a string value from any type of cell (you might also have dates, formulas, etc.) is to use DataFormatter:
import org.apache.poi.ss.usermodel.DataFormatter;
static DataFormatter dataFormatter = new DataFormatter();
static String getStringValue(Cell cell) {
return dataFormatter.formatCellValue(cell);
}
It is especially useful when you want to get exactly the same what is displayed in Excel (e.g numbers rounded to given decimal places).