Default to empty string if CF_STACK unbound

Author: Ryan Moran

bin/run

@@ -93,7 +93,7 @@ function main() {
   local phase
   phase="$(basename "${0}")"
 
-  if [[ "${CF_STACK}" == "cflinuxfs4" ]]; then
+  if [[ "${CF_STACK:-}" == "cflinuxfs4" ]]; then
     if [[ ! -e "${RUBY_DIR}" ]]; then
       mkdir -p "${RUBY_DIR}"