**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 cls._encode(var.total_seconds(), type_=DAT.TIMEDELTA)
            elif isinstance(var, Timezone):
                return cls._encode(str(var.name), type_=DAT.TIMEZONE)
            elif isinstance(var, relativedelta.relativedelta):
                encoded = {k: v for k, v in var.__dict__.items() if not k.startswith("_") and v}
                if var.weekday and var.weekday.n:
                    
                    encoded['weekday'] = [var.weekday.weekday, var.weekday.n]
                elif var.weekday:
                    encoded['weekday'] = [var.weekday.weekday]
                return cls._encode(encoded, type_=DAT.RELATIVEDELTA)
            elif callable(var):
                return str(get_python_source(var))
            elif isinstance(var, set):
                
                return cls._encode([cls._serialize(v) for v in var], type_=DAT.SET)
            elif isinstance(var, tuple):
                
                return cls._encode([cls._serialize(v) for v in var], type_=DAT.TUPLE)
            elif isinstance(var, TaskGroup):
                return SerializedTaskGroup.serialize_task_group(var)
            else:
                log.debug('Cast type %s to str in serialization.', type(var))
                return str(var)
        except Exception:  
            log.error('Failed to stringify.', exc_info=True)
            return FAILED
    
    @classmethod
    def _deserialize(cls, encoded_var: Any) -> Any: