**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 deserialization."""
        
        if cls._is_primitive(encoded_var):
            return encoded_var
        elif isinstance(encoded_var, list):
            return [cls._deserialize(v) for v in encoded_var]
        if not isinstance(encoded_var, dict):
            raise ValueError(f"The encoded_var should be dict and is {type(encoded_var)}")
        var = encoded_var[Encoding.VAR]
        type_ = encoded_var[Encoding.TYPE]
        if type_ == DAT.DICT: