Add option to allow expanders in empty folders

[jralo7]

When issue #344 (to hide expanders for empty folders in list mode, for aesthetics) was implemented, it did not only hide the arrows but also disable expanding them altogether. That caused several problems, including the issues #486, #508, #616, #894 and #3145.

This pull request adds an option to toggle #334's behaviour (optional, by default my changes are disabled).

I think a better solution would be to change the expander icons on empty folders, like ━ or ▷ instead of hiding the ▶. But that would require modifying a lot more stuff, and my attempts have been unsuccessful.

Disclaimers:

• I did use AI, but I reviewed the code and now I'm using it on my computer to see if there are any bugs. Once I find no bugs in some time I'll change "Draft PR" to "Open PR", but feel free to test and merge the draft if you want. I haven't experienced any bugs during a few weeks using it on my computer.
• I closed the previous PR because I messed up my commits while trying to implement the better solution, this PR / branch has cleaner commits.