Pipeline stash and empty directories

[M Bagnall]

I have a set of directories/files that I am stashing during a pipeline build that includes an empty directory. When I unstash the empty directory isn't there. Is this expected behaviour? If so, is there a way to prevent it skipping empty directories? This particular set of files is a third party dependency so I'd rather not have to modify it in any way, e.g. placing a dummy file in the directory.

On a related note, is there documentation available for stash/unstash which describes e.g size limits, how permissions are preserved/not preserved, how symbolic links are handled etc...