**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
"""
        This test verifies that there are no new fields added to BaseOperator. And reminds that
        tests should be added for it.
        """
        base_operator = BaseOperator(task_id="10")
        fields = base_operator.__dict__
        self.assertEqual(
            {
                '_BaseOperator__instantiated': True,
                '_dag': None,
                '_downstream_task_ids': set(),
                '_inlets': [],
                '_log': base_operator.log,
                '_outlets': [],
                '_upstream_task_ids': set(),
                'depends_on_past': False,
                'do_xcom_push': True,
                'email': None,
                'email_on_failure': True,
                'email_on_retry': True,
                'end_date': None,
                'execution_timeout': None,
                'executor_config': {},
                'inlets': [],
                'label': '10',
                'max_retry_delay': None,
                'on_execute_callback': None,