**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
self, serialized_schedule_interval, expected_schedule_interval, expected_n_schedule_interval
    ):
        serialized = {
            "__version": 1,
            "dag": {
                "default_args": {"__type": "dict", "__var": {}},
                "_dag_id": "simple_dag",
                "fileloc": __file__,
                "tasks": [],
                "timezone": "UTC",
                "schedule_interval": serialized_schedule_interval,
            },
        }
        SerializedDAG.validate_schema(serialized)
        dag = SerializedDAG.from_dict(serialized)
        self.assertEqual(dag.schedule_interval, expected_schedule_interval)
        self.assertEqual(dag.normalized_schedule_interval, expected_n_schedule_interval)
    @parameterized.expand(
        [
            (relativedelta(days=-1), {"__type": "relativedelta", "__var": {"days": -1}}),
            (relativedelta(month=1, days=-1), {"__type": "relativedelta", "__var": {"month": 1, "days": -1}}),
            
            (relativedelta(weekday=FR), {"__type": "relativedelta", "__var": {"weekday": [4]}}),
            
            (relativedelta(weekday=FR(2)), {"__type": "relativedelta", "__var": {"weekday": [4, 2]}}),
        ]
    )
    def test_roundtrip_relativedelta(self, val, expected):