**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
"""Helper function of depth first search for serialization.
        The serialization protocol is:
        (1) keeping JSON supported types: primitives, dict, list;
        (2) encoding other types as ``{TYPE: 'foo', VAR: 'bar'}``, the deserialization
            step decode VAR according to TYPE;
        (3) Operator has a special field CLASS to record the original class
            name for displaying in UI.
        """
        try:
            if cls._is_primitive(var):
                
                if isinstance(var, enum.Enum):
                    return var.value
                return var
            elif isinstance(var, dict):
                return cls._encode({str(k): cls._serialize(v) for k, v in var.items()}, type_=DAT.DICT)
            elif isinstance(var, list):
                return [cls._serialize(v) for v in var]
            elif HAS_KUBERNETES and isinstance(var, k8s.V1Pod):
                json_pod = PodGenerator.serialize_pod(var)
                return cls._encode(json_pod, type_=DAT.POD)
            elif isinstance(var, DAG):
                return SerializedDAG.serialize_dag(var)
            elif isinstance(var, BaseOperator):
                return SerializedBaseOperator.serialize_operator(var)
            elif isinstance(var, cls._datetime_types):
                return cls._encode(var.timestamp(), type_=DAT.DATETIME)
            elif isinstance(var, datetime.timedelta):