**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 unfinished_tasks and none_depends_on_past and none_task_concurrency:
                
                are_runnable_tasks = (
                    schedulable_tis
                    or self._are_premature_tis(unfinished_tasks, finished_tasks, session)
                    or changed_tis
                )
        leaf_task_ids = {t.task_id for t in dag.leaves}
        leaf_tis = [ti for ti in tis if ti.task_id in leaf_task_ids]
        
        if not unfinished_tasks and any(leaf_ti.state in State.failed_states for leaf_ti in leaf_tis):
            self.log.error('Marking run %s failed', self)
            self.set_state(State.FAILED)
            if execute_callbacks:
                dag.handle_callback(self, success=False, reason='task_failure', session=session)
            else:
                callback = callback_requests.DagCallbackRequest(
                    full_filepath=dag.fileloc,
                    dag_id=self.dag_id,
                    execution_date=self.execution_date,
                    is_failure_callback=True,
                    msg='task_failure',
                )
        
        elif not unfinished_tasks and all(leaf_ti.state in State.success_states for leaf_ti in leaf_tis):
            self.log.info('Marking run %s successful', self)
            self.set_state(State.SUCCESS)
            if execute_callbacks:
                dag.handle_callback(self, success=True, reason='success', session=session)
            else: