**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
import multiprocessing
import os
import unittest
from datetime import datetime, timedelta, timezone
from glob import glob
from unittest import mock
import pytest
from dateutil.relativedelta import FR, relativedelta
from kubernetes.client import models as k8s
from parameterized import parameterized
from airflow.hooks.base_hook import BaseHook
from airflow.kubernetes.pod_generator import PodGenerator
from airflow.models import DAG, Connection, DagBag, TaskInstance
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.operators.bash import BashOperator
from airflow.security import permissions
from airflow.serialization.json_schema import load_dag_schema_dict
from airflow.serialization.serialized_objects import SerializedBaseOperator, SerializedDAG
from tests.test_utils.mock_operators import CustomOperator, CustomOpLink, GoogleLink
executor_config_pod = k8s.V1Pod(
    metadata=k8s.V1ObjectMeta(name="my-name"),
    spec=k8s.V1PodSpec(
        containers=[
            k8s.V1Container(name="base", volume_mounts=[k8s.V1VolumeMount(name="my-vol", mount_path="/vol/")])
        ]
    ),
)
serialized_simple_dag_ground_truth = {
    "__version": 1,
    "dag": {