**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
if self.state == State.RUNNING:
            return
        duration = self.end_date - self.start_date
        if self.state is State.SUCCESS:
            Stats.timing(f'dagrun.duration.success.{self.dag_id}', duration)
        elif self.state == State.FAILED:
            Stats.timing(f'dagrun.duration.failed.{self.dag_id}', duration)
    @provide_session
    def verify_integrity(self, session: Session = None):
        """
        Verifies the DagRun by checking for removed tasks or tasks that are not in the
        database yet. It will set state to removed or add the task if required.
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        dag = self.get_dag()
        tis = self.get_task_instances(session=session)
        
        task_ids = set()
        for ti in tis:
            task_instance_mutation_hook(ti)
            task_ids.add(ti.task_id)
            task = None
            try:
                task = dag.get_task(ti.task_id)
            except AirflowException:
                if ti.state == State.REMOVED:
                    pass  
                elif self.state is not State.RUNNING and not dag.partial: