fix xlsx parser can not compatible with rich text tags that are not closed

[WingMrL]

Summary

Fix xlsx parser can not compatible with rich text tags that are not closed.

Test plan

Unable to read an xlsx containing a hyperlink described in rich text. Here's the error infos:

error:  TypeError: Cannot create property 'richText' on string ''
    at SharedStringXform.parseClose (/Users/wingmrl/Code/exceljs-demo/node_modules/exceljs/lib/xlsx/xform/strings/shared-string-xform.js:81:40)
    at SharedStringsXform.parseClose (/Users/wingmrl/Code/exceljs-demo/node_modules/exceljs/lib/xlsx/xform/strings/shared-strings-xform.js:111:24)
    at SharedStringsXform.parse (/Users/wingmrl/Code/exceljs-demo/node_modules/exceljs/lib/xlsx/xform/base-xform.js:67:21)

Related to source code (for typings update)

'use strict';

/**
 * Minimum reproducible program
 */

const fs = require('fs');
const stream = require('stream');

const Excel = require('../excel');

const filename = process.argv[2];
const buffer = fs.readFileSync(filename);
const workbook = new Excel.Workbook();

const streamReadable = new stream.Readable();
streamReadable.push(buffer);
streamReadable.push(null);

workbook.xlsx
  .read(streamReadable)
  .then(allWorksheet => {
    console.log('allWorksheet: ', allWorksheet);
  })
  .catch(error => {
    console.error('something went wrong', error.stack);
  });

[zurmokeeper]

@WingMrL Could you please move over here? I'd be happy to merge this feature.