**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
start_date = Column(UtcDateTime, default=timezone.utcnow)
    end_date = Column(UtcDateTime)
    _state = Column('state', String(50), default=State.RUNNING)
    run_id = Column(String(ID_LEN))
    creating_job_id = Column(Integer)
    external_trigger = Column(Boolean, default=True)
    run_type = Column(String(50), nullable=False)
    conf = Column(PickleType)
    
    last_scheduling_decision = Column(UtcDateTime)
    dag_hash = Column(String(32))
    dag = None
    __table_args__ = (
        Index('dag_id_state', dag_id, _state),
        UniqueConstraint('dag_id', 'execution_date'),
        UniqueConstraint('dag_id', 'run_id'),
        Index('idx_last_scheduling_decision', last_scheduling_decision),
    )
    task_instances = relationship(
        TI,
        primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == execution_date),  
        foreign_keys=(dag_id, execution_date),
        backref=backref('dag_run', uselist=False),
    )
    DEFAULT_DAGRUNS_TO_EXAMINE = airflow_conf.getint(
        'scheduler',
        'max_dagruns_per_loop_to_schedule',
        fallback=20,
    )
    def __init__(