**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
schedulable_tis: List[TI] = []
        changed_tis = False
        tis = list(self.get_task_instances(session=session, state=State.task_states + (State.SHUTDOWN,)))
        self.log.debug("number of tis tasks for %s: %s task(s)", self, len(tis))
        for ti in tis:
            ti.task = self.get_dag().get_task(ti.task_id)
        unfinished_tasks = [t for t in tis if t.state in State.unfinished]
        finished_tasks = [t for t in tis if t.state in State.finished]
        if unfinished_tasks:
            scheduleable_tasks = [ut for ut in unfinished_tasks if ut.state in SCHEDULEABLE_STATES]
            self.log.debug("number of scheduleable tasks for %s: %s task(s)", self, len(scheduleable_tasks))
            schedulable_tis, changed_tis = self._get_ready_tis(scheduleable_tasks, finished_tasks, session)
        return TISchedulingDecision(
            tis=tis,
            schedulable_tis=schedulable_tis,
            changed_tis=changed_tis,
            unfinished_tasks=unfinished_tasks,
            finished_tasks=finished_tasks,
        )
    def _get_ready_tis(