Added check for "unset" variables

/cc @netomi

with this you get

./download-github-release.sh: line 62: REPO: unbound variable

this is due to the use of getopts and setting variables only when an option is provided. I did not find a clean way to workaround, so I removed the -u option.

Solution is to use the shell parameter expansion in this case:

if [ -z "${REPO:-}" ] || [ -z "${VERSION:-}" ] || [ -z "${ARTIFACT:-}" ]; then
    usage
fi

fixed on main.

closed