**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
"""
        Additional Properties are disabled on DAGs. This test verifies that all the
        keys in DAG.get_serialized_fields are listed in Schema definition.
        """
        dag_schema: dict = load_dag_schema_dict()["definitions"]["dag"]["properties"]
        
        ignored_keys: set = {"is_subdag", "tasks"}
        dag_params: set = set(dag_schema.keys()) - ignored_keys
        self.assertEqual(set(DAG.get_serialized_fields()), dag_params)
    def test_operator_subclass_changing_base_defaults(self):
        assert (
            BaseOperator(task_id='dummy').do_xcom_push is True
        ), "Precondition check! If this fails the test won't make sense"
        class MyOperator(BaseOperator):
            def __init__(self, do_xcom_push=False, **kwargs):
                super().__init__(**kwargs)
                self.do_xcom_push = do_xcom_push
        op = MyOperator(task_id='dummy')
        assert op.do_xcom_push is False
        blob = SerializedBaseOperator.serialize_operator(op)
        serialized_op = SerializedBaseOperator.deserialize_operator(blob)
        assert serialized_op.do_xcom_push is False
    def test_no_new_fields_added_to_base_operator(self):