fix install script without tailscale

[juliendsv]

Running bash install.sh --no-tailscale (or any invocation that results in an empty SETUP_ARGS array) fails with:

  install.sh: line 121: SETUP_ARGS[@]: unbound variable

This is caused by set -u treating an empty array's [@] expansion as unbound — a known bash quirk.

Fix

Use the ${var+word} conditional expansion pattern on line 121, which
only expands the array when it has elements and produces nothing when it
is empty:

  ./setup.sh "${SETUP_ARGS[@]+"${SETUP_ARGS[@]}"}"

This is the same pattern already used on line 100 for the filtered
array.