Need to use a default value

Author: huydhn

.ci/scripts/utils.sh

@@ -20,7 +20,7 @@ install_executorch() {
   which pip
   # Install executorch, this assumes that Executorch is checked out in the
   # current directory.
-  if [[ $1 == "use-pt-pinned-commit" ]]; then
+  if [[ "${1:-}" == "use-pt-pinned-commit" ]]; then
     ./install_requirements.sh --pybind xnnpack --use-pt-pinned-commit
   else
     ./install_requirements.sh --pybind xnnpack