**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
tests/serialization/test_dag_serialization.py
`airflow/serialization/schema.json` - they should have correct type defined there.
 Note that we do not support versioning yet so you should only add optional fields to BaseOperator.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
                         """,
        )
    def test_task_group_serialization(self):
        """
        Test TaskGroup serialization/deserialization.
        """
        from airflow.operators.dummy_operator import DummyOperator
        from airflow.utils.task_group import TaskGroup
        execution_date = datetime(2020, 1, 1)
        with DAG("test_task_group_serialization", start_date=execution_date) as dag:
            task1 = DummyOperator(task_id="task1")
            with TaskGroup("group234") as group234:
                _ = DummyOperator(task_id="task2")
                with TaskGroup("group34") as group34:
                    _ = DummyOperator(task_id="task3")
                    _ = DummyOperator(task_id="task4")
            task5 = DummyOperator(task_id="task5")
            task1 >> group234
            group34 >> task5
        dag_dict = SerializedDAG.to_dict(dag)
        SerializedDAG.validate_schema(dag_dict)
        json_dag = SerializedDAG.from_json(SerializedDAG.to_json(dag))
        self.validate_deserialized_dag(json_dag, dag)
        serialized_dag = SerializedDAG.deserialize_dag(SerializedDAG.serialize_dag(dag))
        assert serialized_dag.task_group.children
        assert serialized_dag.task_group.children.keys() == dag.task_group.children.keys()
        def check_task_group(node):