fix #2751 - Csv reading - cells filled with spaces only are converted to 0

[tlgman]

Summary

Fix #2751

Test plan

const wb = new ExcelJS.Workbook();
wb.csv.read(Stream.Readable.from('firstValue;    ;secondValue', { parserOptions: { delimiter: ';' } })
const ws = wb.worksheets[templateInfo.worksheet];

const row1 = ws.getRow(1);
const cell12 = row1.getCell(2);
console.log('cell: 1,2: ', cell12.value); // Displays:  '    ' 

Related to source code (for typings update)

(https://github.com/exceljs/exceljs/blob/master/lib/csv/csv.js#L58)