**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
for k, v in encoded_op.items():
            if k == "_downstream_task_ids":
                v = set(v)
            elif k == "subdag":
                v = SerializedDAG.deserialize_dag(v)
            elif k in {"retry_delay", "execution_timeout"}:
                v = cls._deserialize_timedelta(v)
            elif k in encoded_op["template_fields"]:
                pass
            elif k.endswith("_date"):
                v = cls._deserialize_datetime(v)
            elif k == "_operator_extra_links":
                op_predefined_extra_links = cls._deserialize_operator_extra_links(v)
                
                op_predefined_extra_links.update(op_extra_links_from_plugin)
                v = list(op_predefined_extra_links.values())
                k = "operator_extra_links"
            elif k in cls._decorated_fields or k not in op.get_serialized_fields():
                v = cls._deserialize(v)
            
            setattr(op, k, v)
        for k in op.get_serialized_fields() - encoded_op.keys() - cls._CONSTRUCTOR_PARAMS.keys():
            setattr(op, k, None)
        
        for field in op.template_fields:
            if not hasattr(op, field):
                setattr(op, field, None)
        return op
    @classmethod
    def _is_excluded(cls, var: Any, attrname: str, op: BaseOperator):