Fixes crash when no shared string index present.

[nramsbottom]

I received an Excel file that could not be opened because it caused a crash. After some debugging I determined that it is because the v element within a c element doesn't have a value and the existing code expects there to always be a value.

The file was generated by an external vendor, so I cannot alter how it is generated. The file can be opened by Excel and LibreOffice Calc with no issues.

I have produced a sample of the "broken" file here.

Book1.xlsx

This fix ensures that the shared string index is read as a string and parsed safely and the appropriate content placed into the cell.

[nramsbottom]

using OfficeOpenXml;

using var fileStream = File.OpenRead(@"..\..\..\..\Book1.xlsx");
using var package = new ExcelPackage(fileStream);

var workSheet = package.Workbook.Worksheets.FirstOrDefault();
var rowCount = workSheet.Dimension.End.Row;

Console.WriteLine($"{rowCount} rows.");
Console.ReadKey();

Sample code I used to test.