Create an empty default welcome file

_Originally posted by @ann0see in https://github.com/jamulussoftware/jamulus/pull/3194#pullrequestreview-1970350884_

I'd not do it this way.
Modify the files in this folder instead: https://github.com/jamulussoftware/jamulus/tree/main/linux/debian
Add an empty .html file to the directory.

Add the empty file here: https://github.com/jamulussoftware/jamulus/blob/main/linux/debian/jamulus.install and here: https://github.com/jamulussoftware/jamulus/blob/main/linux/debian/jamulus-headless.install

Maybe you'd also need to edit https://github.com/jamulussoftware/jamulus/blob/main/linux/deploy_deb.sh

Start by reading linux/debian/jamulus.install and linux/debian/jamulus-headless.install, then inspect linux/deploy_deb.sh and the existing Debian package layout. Add the requested empty HTML file entries and verify that both package variants build with the expected default welcome file included.

Written by the indexing model from the issue text.