**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
"""A JSON serializable representation of operator.
    All operators are casted to SerializedBaseOperator after deserialization.
    Class specific attributes used by UI are move to object attributes.
    """
    _decorated_fields = {'executor_config'}
    _CONSTRUCTOR_PARAMS = {
        k: v.default
        for k, v in signature(BaseOperator.__init__).parameters.items()
        if v.default is not v.empty
    }
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        
        
        self._task_type = 'BaseOperator'
        
        self.ui_color = BaseOperator.ui_color
        self.ui_fgcolor = BaseOperator.ui_fgcolor
        self.template_fields = BaseOperator.template_fields
        self.operator_extra_links = BaseOperator.operator_extra_links
    @property
    def task_type(self) -> str: