[BUG] FsWorkbook can be saved to file with no worksheet present

[omaus]

Describe the bug
It is possible to save an FsWorkbook with no worksheets attached. This is critical since an XLSX spreadsheet file always needs to have at least 1 worksheet with a name given. Else it is not valid and, e.g., MS Excel will tell you it is corrupted when trying to open it.

To Reproduce
Steps to reproduce the behavior:
1.

#r "nuget: FsSpreadsheet"
#r "nuget: FsSpreadsheet.ExcelIO"

open FsSpreadsheet
open FsSpreadsheet.ExcelIO

let path = "test.xlsx"

let wb = new FsWorkbook()

FsWorkbook.toFile path wb

2. Open in MS Excel

Expected behavior
This is up to debate.
Maybe throw an error when trying to save such a workbook? Or at least give a warning? Is this even a bug at all or do we want to give the library user the freedom to artificially produce invalid XLSX files?
@HLWeil

[HLWeil]

Now the funciton fails if no worksheet is present.