[workflow] fix workflow user metadata return when None is given (#19356)

## Why are these changes needed?

Quick fix for metadata put. Currently when workflow-level metadata is not given, it will output `null` to `user_run_metadata.json`, this fix will make it output `{}`.
## Related issue number

original issue: https://github.com/ray-project/ray/issues/17090
original PR: https://github.com/ray-project/ray/pull/19195

python/ray/workflow/execution.py

@@ -37,6 +37,7 @@ def run(entry_workflow: Workflow,
                 raise ValueError("metadata values must be JSON serializable, "
                                  "however '{}' has a value whose {}.".format(
                                      k, e))
+    metadata = metadata or {}
 
     store = get_global_storage()
     assert ray.is_initialized()

python/ray/workflow/tests/test_metadata_put.py

@@ -86,6 +86,25 @@ def test_workflow_runtime_metadata(workflow_start_regular):
     assert "end_time" in postrun_meta
 
 
+def test_no_user_metadata(workflow_start_regular):
+
+    workflow_id = "simple"
+    step_name = "simple_step"
+
+    @workflow.step(name=step_name)
+    def simple():
+        return 0
+
+    simple.step().run(workflow_id)
+
+    checkpointed_user_step_metadata = get_metadata(
+        [workflow_id, "steps", step_name, workflow_storage.STEP_USER_METADATA])
+    checkpointed_user_run_metadata = get_metadata(
+        [workflow_id, workflow_storage.WORKFLOW_USER_METADATA])
+    assert {} == checkpointed_user_step_metadata
+    assert {} == checkpointed_user_run_metadata
+
+
 def test_all_metadata(workflow_start_regular):
 
     user_step_metadata = {"k1": "v1"}