**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
self,
        scheduleable_tasks: List[TI],
        finished_tasks: List[TI],
        session: Session,
    ) -> Tuple[List[TI], bool]:
        old_states = {}
        ready_tis: List[TI] = []
        changed_tis = False
        if not scheduleable_tasks:
            return ready_tis, changed_tis
        
        for st in scheduleable_tasks:
            old_state = st.state
            if st.are_dependencies_met(
                dep_context=DepContext(flag_upstream_failed=True, finished_tasks=finished_tasks),
                session=session,
            ):
                ready_tis.append(st)
            else:
                old_states[st.key] = old_state
        
        tis_filter = TI.filter_for_tis(old_states.keys())
        if tis_filter is not None:
            fresh_tis = session.query(TI).filter(tis_filter).all()
            changed_tis = any(ti.state != old_states[ti.key] for ti in fresh_tis)
        return ready_tis, changed_tis
    def _are_premature_tis(