**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/baseoperator.py
self,
        upstream: bool = False,
        found_descendants: Optional[Set[str]] = None,
    ) -> Set[str]:
        """Get a flat set of relatives' ids, either upstream or downstream."""
        if not self._dag:
            return set()
        if not found_descendants:
            found_descendants = set()
        relative_ids = self.get_direct_relative_ids(upstream)
        for relative_id in relative_ids:
            if relative_id not in found_descendants:
                found_descendants.add(relative_id)
                relative_task = self._dag.task_dict[relative_id]
                relative_task.get_flat_relative_ids(upstream, found_descendants)
        return found_descendants
    def get_flat_relatives(self, upstream: bool = False):
        """Get a flat list of relatives, either upstream or downstream."""
        if not self._dag:
            return set()
        from airflow.models.dag import DAG
        dag: DAG = self._dag
        return list(map(lambda task_id: dag.task_dict[task_id], self.get_flat_relative_ids(upstream)))
    def run(