Guard against incomplete workbooks

As seen in this [SO](https://stackoverflow.com/q/77357576/12340029).

``` r
library(openxlsx2)
wb <- wb_workbook()$add_worksheet()
wb$workbook.xml.rels <- character()
wb$save(file = temp_xlsx())
#> Error in `$<-.data.frame`(`*tmp*`, tmpDirPartName, value = "/var/folders/p7/t9znbbgj2_ndlz6vbxmgh7zm0000gn/T//RtmpITgInh/workbookTemp_f4ae7f3030e1/xl/"): replacement has 1 row, data has 0
```