Let PlayIterator.add_tasks accept empty task lists

PlayIterator.add_tasks raised an error when trying to add an empty task
list.  This was the root cause of ansible issue #13370.

lib/ansible/executor/play_iterator.py

@@ -399,6 +399,9 @@ class PlayIterator:
         if state.fail_state != self.FAILED_NONE:
             return state
 
+        if not task_list:
+            return state
+
         if state.run_state == self.ITERATING_TASKS:
             if state.tasks_child_state:
                 state.tasks_child_state = self._insert_tasks_into_state(state.tasks_child_state, task_list)