**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
"default_args": {
            "__type": "dict",
            "__var": {
                "depends_on_past": False,
                "retries": 1,
                "retry_delay": {"__type": "timedelta", "__var": 300.0},
            },
        },
        "start_date": 1564617600.0,
        '_task_group': {
            '_group_id': None,
            'prefix_group_id': True,
            'children': {'bash_task': ('operator', 'bash_task'), 'custom_task': ('operator', 'custom_task')},
            'tooltip': '',
            'ui_color': 'CornflowerBlue',
            'ui_fgcolor': '#000',
            'upstream_group_ids': [],
            'downstream_group_ids': [],
            'upstream_task_ids': [],
            'downstream_task_ids': [],
        },
        "is_paused_upon_creation": False,
        "_dag_id": "simple_dag",
        "fileloc": None,
        "tasks": [
            {
                "task_id": "bash_task",
                "owner": "airflow",
                "retries": 1,
                "retry_delay": 300.0,