**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
return self._task_type
    @task_type.setter
    def task_type(self, task_type: str):
        self._task_type = task_type
    @classmethod
    def serialize_operator(cls, op: BaseOperator) -> dict:
        """Serializes operator into a JSON object."""
        serialize_op = cls.serialize_to_json(op, cls._decorated_fields)
        serialize_op['_task_type'] = op.__class__.__name__
        serialize_op['_task_module'] = op.__class__.__module__
        if op.operator_extra_links:
            serialize_op['_operator_extra_links'] = cls._serialize_operator_extra_links(
                op.operator_extra_links
            )
        
        
        if op.template_fields:
            for template_field in op.template_fields:
                value = getattr(op, template_field, None)
                if not cls._is_excluded(value, template_field, op):
                    serialize_op[template_field] = serialize_template_field(value)
        return serialize_op
    @classmethod
    def deserialize_operator(cls, encoded_op: Dict[str, Any]) -> BaseOperator: