writeDataTable doesn't handle NULL data.frames or data.tables

[ScThSz]

Hello,

I've come across an issue in using openxlsx::writeDataTable with null data.frames and data.tables. I'm wondering if I'm missing something there but the documentation doesn't seem to mention anything.

Expected Behavior

I have a set of data.tables/data.frames that I want to write to an Excel file, some of them can be empty.
If the dt/df is empty I would expect the excel tab to be named according to the "sheet" argument and that the content is empty.

Actual Behavior

Currently this seems to raise an Index out of bounds error coming from:
https://github.com/ycphs/openxlsx/blob/master/R/workbook_write_data.R#L37
as "nCols" will be 0 and 1:nCols will result in an integer vector of length 2 (1, 0).
Maybe seq_len or something could fix that problem?

Steps to Reproduce the Problem

`
library(openxlsx)
library(data.table)

wb <- openxlsx::createWorkbook()
openxlsx::addWorksheet(wb, sheetName = "test")
openxlsx::writeDataTable(wb, sheet = "test", x = data.table::data.table())
openxlsx::writeDataTable(wb, sheet = "test", x = data.frame()) # doesn't seem to work with base dataframes either
`

sessionInfo()

R version 4.1.2 (2021-11-01)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux 10 (buster)

Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/openblas/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/libopenblasp-r0.3.5.so

locale:
[1] LC_CTYPE=de_DE.UTF-8 LC_NUMERIC=C LC_TIME=de_DE.UTF-8 LC_COLLATE=de_DE.UTF-8
[5] LC_MONETARY=de_DE.UTF-8 LC_MESSAGES=de_DE.UTF-8 LC_PAPER=de_DE.UTF-8 LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] data.table_1.14.2 openxlsx_4.2.4 devtools_2.4.2 usethis_2.1.3

loaded via a namespace (and not attached):
[1] Rcpp_1.0.7 rstudioapi_0.13 magrittr_2.0.1 pkgload_1.2.3 R6_2.5.1 rlang_1.0.2
[7] fastmap_1.1.0 tools_4.1.2 pkgbuild_1.2.0 sessioninfo_1.2.1 cli_3.3.0 withr_2.5.0
[13] ellipsis_0.3.2 remotes_2.4.1 rprojroot_2.0.2 lifecycle_1.0.1 crayon_1.4.2 brio_1.1.2
[19] zip_2.2.0 processx_3.5.2 purrr_0.3.4 callr_3.7.0 fs_1.5.0 ps_1.6.0
[25] testthat_3.1.4 memoise_2.0.0 glue_1.6.2 cachem_1.0.6 stringi_1.7.6 compiler_4.1.2
[31] desc_1.4.0 prettyunits_1.1.1

• Version of openxlsx: 4.2.4
• Version of R: 4.1.2
• Version of data.table: 1.14.2