**Apache Airflow version**:
v2.0.0

**What happened**:

Custom operators inheriting from DummyOperator are go in to a scheduled state, where as DummyOperator itself is set straight to success.

**What you expected to happen**:

All operators should be successfully executed.

**How to reproduce it**:

```py
from datetime import datetime
from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator

class MyJob(DummyOperator):
    template_fields_renderers = {
        "body": "json"
    }
    template_fields = ("body",)

    def __init__(self, body, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.body = body

with DAG(
    dag_id="test",
    schedule_interval=None,
    start_date=datetime(2020, 8, 13),
) as dag:
    MyJob(task_id="aaaa", body={"aaa": "bbb"})

```

**Anything else we need to know**:
This is a known problem that we will fix, see: https://github.com/apache/airflow/pull/10956#discussion_r502492775
airflow/serialization/serialized_objects.py
dag._task_group = SerializedTaskGroup.deserialize_task_group(  
                encoded_dag["_task_group"], None, dag.task_dict
            )
        else:
            
            
            dag._task_group = TaskGroup.create_root(dag)
            for task in dag.tasks:
                dag.task_group.add(task)
        
        keys_to_set_none = dag.get_serialized_fields() - encoded_dag.keys() - cls._CONSTRUCTOR_PARAMS.keys()
        for k in keys_to_set_none:
            setattr(dag, k, None)
        setattr(dag, 'full_filepath', dag.fileloc)
        for task in dag.task_dict.values():
            task.dag = dag
            serializable_task: BaseOperator = task
            for date_attr in ["start_date", "end_date"]:
                if getattr(serializable_task, date_attr) is None:
                    setattr(serializable_task, date_attr, getattr(dag, date_attr))
            if serializable_task.subdag is not None:
                setattr(serializable_task.subdag, 'parent_dag', dag)
                serializable_task.subdag.is_subdag = True
            for task_id in serializable_task.downstream_task_ids:
                
                
                dag.task_dict[task_id]._upstream_task_ids.add(serializable_task.task_id)
        return dag
    @classmethod
    def to_dict(cls, var: Any) -> dict: