Reading from excel file with blank cells to 2d array

Question

I have a following code that reads logins and passwords from xls file starting from the second row(it skips column names) and writes it into a 2d array. But it only works if the sheet doesn't have blank cells in any of the rows. What should i do to make it work with empty cells?

private static Object[][] getUsersFromXls(String sheetName) {
        final File excelFile = new File("src//resources//TestData.xls");
        FileInputStream fileInputStream;

        try {
            fileInputStream = new FileInputStream(excelFile);
            workbook = new HSSFWorkbook(fileInputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }

        sheet = workbook.getSheet(sheetName);

        final int numberOfRows = sheet.getLastRowNum();
        final int numberOfColumns = sheet.getRow(0).getLastCellNum();

        final String[][] xlsData = new String[numberOfRows][numberOfColumns];
        String cellValue;

        for (int i = 1; i <= numberOfRows; i++) {
            final HSSFRow row = sheet.getRow(i);

            for (int j = 0; j < numberOfColumns; j++) {
                final HSSFCell cell = row.getCell(j);
                final int cellType = cell.getCellType();

                if (cellType == HSSFCell.CELL_TYPE_FORMULA) {
                    throw new RuntimeException("Cannot process a formula. Please change field to result of formula.");
                } else {
                    cellValue = String.valueOf(cell);
                    xlsData[i - 1][j] = cellValue;
                }
            }
        }
        return xlsData;
    }