Empty sheet creates invalid xlsx file

[atymic]

If you try to export an empty sheet, we end up with a invalid XLSX file.
File is attached that is produced

 $exporter = new FastExcel(new SheetCollection([
                'A' => collect([['a' => 'b']]),
                'B' => collect(),
            ]

failed.xlsx

[atymic]

Adding an empty row resolves the issue