make task result data optional key

inventory_provider/tasks/worker.py

@@ -408,11 +408,13 @@ def launch_refresh_cache_all(config):
 
 def check_task_status(task_id):
     r = AsyncResult(task_id, app=app)
-    return {
+    result = {
         'id': task_id,
         'status': r.status,
         'exception': r.status in states.EXCEPTION_STATES,
         'ready': r.status in states.READY_STATES,
         'success': r.status == states.SUCCESS,
-        'result': r.result
     }
+    if r.result:
+        result['result'] = r.result
+    return result