### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
if isinstance(child, AbstractOperator):
                    if child.task_id in dag.task_dict:
                        task = copied.children[child.task_id] = dag.task_dict[child.task_id]
                        task.task_group = proxy
                    else:
                        copied.used_group_ids.discard(child.task_id)
                else:
                    filtered_child = filter_task_group(child, proxy)
                    
                    if filtered_child.children:
                        copied.children[child.group_id] = filtered_child
            return copied
        dag._task_group = filter_task_group(self.task_group, None)
        
        
        subdag_task_groups = dag.task_group.get_task_group_dict()
        for group in subdag_task_groups.values():
            group.upstream_group_ids.intersection_update(subdag_task_groups)
            group.downstream_group_ids.intersection_update(subdag_task_groups)
            group.upstream_task_ids.intersection_update(dag.task_dict)
            group.downstream_task_ids.intersection_update(dag.task_dict)
        for t in dag.tasks:
            
            
            t.upstream_task_ids.intersection_update(dag.task_dict)
            t.downstream_task_ids.intersection_update(dag.task_dict)
        if len(dag.tasks) < len(self.tasks):
            dag.partial = True
        return dag
    def has_task(self, task_id: str):