Fix setup.sh crash on Mac with empty gitignore array

The `set -u` (nounset) flag in setup.sh causes `${_HIDDEN_GITIGNORES[@]}`
to fail with "unbound variable" when no parent .gitignore with `*` is
found (common on Mac where the install is not inside a Python venv).

Use the `${arr[@]+"${arr[@]}"}` idiom to safely expand empty arrays
under nounset mode.

studio/setup.sh

@@ -137,7 +137,7 @@ while [ "$_dir" != "/" ]; do
 done
 
 _restore_gitignores() {
-    for _gi in "${_HIDDEN_GITIGNORES[@]}"; do
+    for _gi in "${_HIDDEN_GITIGNORES[@]+"${_HIDDEN_GITIGNORES[@]}"}"; do
         mv "${_gi}._twbuild" "$_gi" 2>/dev/null || true
     done
 }