**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/models/dagrun.py
"""
        Returns the task instance specified by task_id for this dag run
        :param task_id: the task id
        :type task_id: str
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        ti = (
            session.query(TI)
            .filter(TI.dag_id == self.dag_id, TI.execution_date == self.execution_date, TI.task_id == task_id)
            .first()
        )
        return ti
    def get_dag(self):
        """
        Returns the Dag associated with this DagRun.
        :return: DAG
        """
        if not self.dag:
            raise AirflowException(f"The DAG (.dag) for {self} needs to be set")
        return self.dag
    @provide_session
    def get_previous_dagrun(self, state: Optional[str] = None, session: Session = None) -> Optional['DagRun']: