**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
airflow/serialization/serialized_objects.py
"""Serializes a DAG into a JSON object."""
        serialize_dag = cls.serialize_to_json(dag, cls._decorated_fields)
        serialize_dag["tasks"] = [cls._serialize(task) for _, task in dag.task_dict.items()]
        serialize_dag['_task_group'] = SerializedTaskGroup.serialize_task_group(dag.task_group)
        return serialize_dag
    @classmethod
    def deserialize_dag(cls, encoded_dag: Dict[str, Any]) -> 'SerializedDAG':
        """Deserializes a DAG from a JSON object."""
        dag = SerializedDAG(dag_id=encoded_dag['_dag_id'])
        for k, v in encoded_dag.items():
            if k == "_downstream_task_ids":
                v = set(v)
            elif k == "tasks":
                v = {task["task_id"]: SerializedBaseOperator.deserialize_operator(task) for task in v}
                k = "task_dict"
            elif k == "timezone":
                v = cls._deserialize_timezone(v)
            elif k in {"dagrun_timeout"}:
                v = cls._deserialize_timedelta(v)
            elif k.endswith("_date"):
                v = cls._deserialize_datetime(v)
            elif k in cls._decorated_fields:
                v = cls._deserialize(v)
            
            setattr(dag, k, v)
        
        
        if "_task_group" in encoded_dag: