**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
'on_failure_callback': None,
                'on_retry_callback': None,
                'on_success_callback': None,
                'outlets': [],
                'owner': 'airflow',
                'params': {},
                'pool': 'default_pool',
                'pool_slots': 1,
                'priority_weight': 1,
                'queue': 'default',
                'resources': None,
                'retries': 0,
                'retry_delay': timedelta(0, 300),
                'retry_exponential_backoff': False,
                'run_as_user': None,
                'sla': None,
                'start_date': None,
                'subdag': None,
                'task_concurrency': None,
                'task_id': '10',
                'trigger_rule': 'all_success',
                'wait_for_downstream': False,
                'weight_rule': 'downstream',
            },
            fields,
            """
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     ACTION NEEDED! PLEASE READ THIS CAREFULLY AND CORRECT TESTS CAREFULLY
 Some fields were added to the BaseOperator! Please add them to the list above and make sure that
 you add support for DAG serialization - you should add the field to