**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 {k: cls._deserialize(v) for k, v in var.items()}
        elif type_ == DAT.DAG:
            return SerializedDAG.deserialize_dag(var)
        elif type_ == DAT.OP:
            return SerializedBaseOperator.deserialize_operator(var)
        elif type_ == DAT.DATETIME:
            return pendulum.from_timestamp(var)
        elif type_ == DAT.POD:
            if not HAS_KUBERNETES:
                raise RuntimeError("Cannot deserialize POD objects without kubernetes libraries installed!")
            pod = PodGenerator.deserialize_model_dict(var)
            return pod
        elif type_ == DAT.TIMEDELTA:
            return datetime.timedelta(seconds=var)
        elif type_ == DAT.TIMEZONE:
            return Timezone(var)
        elif type_ == DAT.RELATIVEDELTA:
            if 'weekday' in var:
                var['weekday'] = relativedelta.weekday(*var['weekday'])  
            return relativedelta.relativedelta(**var)
        elif type_ == DAT.SET:
            return {cls._deserialize(v) for v in var}
        elif type_ == DAT.TUPLE:
            return tuple([cls._deserialize(v) for v in var])
        else:
            raise TypeError(f'Invalid type {type_!s} in deserialization.')
    _deserialize_datetime = pendulum.from_timestamp
    _deserialize_timezone = pendulum.tz.timezone
    @classmethod
    def _deserialize_timedelta(cls, seconds: int) -> datetime.timedelta: