**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/serialization/serialized_objects.py
"""A JSON serializable representation of TaskGroup."""
    @classmethod
    def serialize_task_group(cls, task_group: TaskGroup) -> Optional[Union[Dict[str, Any]]]:
        """Serializes TaskGroup into a JSON object."""
        if not task_group:
            return None
        serialize_group = {
            "_group_id": task_group._group_id,  
            "prefix_group_id": task_group.prefix_group_id,
            "tooltip": task_group.tooltip,
            "ui_color": task_group.ui_color,
            "ui_fgcolor": task_group.ui_fgcolor,
            "children": {
                label: (DAT.OP, child.task_id)
                if isinstance(child, BaseOperator)
                else (DAT.TASK_GROUP, SerializedTaskGroup.serialize_task_group(child))
                for label, child in task_group.children.items()
            },
            "upstream_group_ids": cls._serialize(list(task_group.upstream_group_ids)),
            "downstream_group_ids": cls._serialize(list(task_group.downstream_group_ids)),
            "upstream_task_ids": cls._serialize(list(task_group.upstream_task_ids)),
            "downstream_task_ids": cls._serialize(list(task_group.downstream_task_ids)),
        }
        return serialize_group
    @classmethod
    def deserialize_task_group(