New issue by ahesford on void-packages repository

https://github.com/void-linux/void-packages/issues/39797

Description:
The additon of `MANPATH` to `/etc/profile.d/texlive.sh` in https://github.com/void-linux/void-packages/commit/37e10282c3e7af286bea3aabb7ad4445ccd7de77 breaks completions for `man` in `zsh`. Previously, `MANPATH` was not set in the environment, which means that `zsh` looks for completions in default locations and finds everything in `/usr/share/man`. However, the new behavior results in `MANPATH=:/opt/texlive/2022/texmf-dist/doc/man`, which means `zsh` will **only** search in the listed directory. Note, also, the leading `:` which results from failing to check whether `MANPATH` is empty before appending the TeXLive path.

I'm not sure what the right fix is, but I guess my prefence would be something like
```sh
[ -n "${MANPATH}" ] && MANPATH="${MANPATH}:/opt/texlive/2022/texmf-dist/doc/man"
```
This will leave `MANPATH` blank by default and leave TL out of the search path, but I suspect that it's generally preferrable to find the system manual pages via tab completion instead of only showing TL pages. The user can always opt into TL manpages by defining a non-zero `MANPATH` in, *e.g.*, `/etc/environment` or a preceding file in `/etc/profile.d`.

cc: @leahneukirchen @balejk