**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
tests/serialization/test_dag_serialization.py
"""Serialisation / deserialisation continues to work without kubernetes installed"""
    def mock__import__(name, globals_=None, locals_=None, fromlist=(), level=0):
        if level == 0 and name.partition('.')[0] == 'kubernetes':
            raise ImportError("No module named 'kubernetes'")
        return importlib.__import__(name, globals=globals_, locals=locals_, fromlist=fromlist, level=level)
    with mock.patch('builtins.__import__', side_effect=mock__import__) as import_mock:
        
        
        spec = importlib.util.find_spec("airflow.serialization.serialized_objects")
        module = importlib.util.module_from_spec(spec)
        spec.loader.exec_module(module)
        
        
        imported_airflow = {
            c.args[0].split('.', 2)[1] for c in import_mock.call_args_list if c.args[0].startswith("airflow.")
        }
        assert "kubernetes" not in imported_airflow
        
        pod_override = {
            '__type': 'k8s.V1Pod',
            '__var': PodGenerator.serialize_pod(executor_config_pod),
        }
        with pytest.raises(RuntimeError):
            module.BaseSerialization.from_dict(pod_override)
        
        module.SerializedDAG.to_dict(make_simple_dag()["simple_dag"])