fix: split resume bash expansions so empty steps-to-rerun doesn't break CLI

When METAFLOW_RESUME_STEPS_TO_RERUN is empty, the single ${VAR:+...}
expansion produced '--resume-steps-to-rerun' with no value, causing
Click to exit with code 2. Split into two separate expansions so
--resume-steps-to-rerun only appears when the value is non-empty.

Author: Nissan Pow

metaflow/plugins/aws/step_functions/step_functions.py

@@ -1072,7 +1072,8 @@ def _step_cli(self, node, paths, code_package_url, user_code_retries):
         step_cmd = " ".join(entrypoint + top_level + step)
         resume_opts = (
             "${METAFLOW_RESUME_ORIGIN_RUN_ID:+"
-            "--resume-origin-run-id $METAFLOW_RESUME_ORIGIN_RUN_ID "
+            "--resume-origin-run-id $METAFLOW_RESUME_ORIGIN_RUN_ID}"
+            " ${METAFLOW_RESUME_STEPS_TO_RERUN:+"
             "--resume-steps-to-rerun $METAFLOW_RESUME_STEPS_TO_RERUN}"
         )
         cmds.append("%s %s" % (step_cmd, resume_opts))