BigDecimal读取到c.t为‘e’时，解析失败

[CarlJia]

版本：eec:0.4.11

    /**
     * Get {@link java.math.BigDecimal} value
     *
     * @param c the {@link Cell}
     * @return BigDecimal
     */
    protected BigDecimal getDecimal(Cell c) {
        BigDecimal bd;
        switch (c.t) {
            case DOUBLE:
                bd = BigDecimal.valueOf(c.dv);
                break;
            case NUMERIC:
                bd = BigDecimal.valueOf(c.nv);
                break;
            case LONG:
                bd = BigDecimal.valueOf(c.lv);
                break;
            case SST:
                if (c.sv == null) {
                    c.setSv(sst.get(c.nv));
                }
                // @Mark:=>There is no missing `break`, this is normal logic here
            case INLINESTR:
                bd = new BigDecimal(c.sv);
                break;
            default: throw new UncheckedTypeException("Can't convert cell value to java.math.BigDecimal");
        }
        return bd;
    }

获取decimal 值时没有解析空数据，解析报错

[wangguanquan]

#180