**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/models/baseoperator.py
"""Getting the content of files for template_field / template_ext"""
        if self.template_ext:  
            for field in self.template_fields:
                content = getattr(self, field, None)
                if content is None:  
                    continue
                elif isinstance(content, str) and any(content.endswith(ext) for ext in self.template_ext):
                    env = self.get_template_env()
                    try:
                        setattr(self, field, env.loader.get_source(env, content)[0])
                    except Exception as e:  
                        self.log.exception(e)
                elif isinstance(content, list):
                    env = self.dag.get_template_env()
                    for i in range(len(content)):  
                        if isinstance(content[i], str) and any(
                            content[i].endswith(ext) for ext in self.template_ext
                        ):
                            try:
                                content[i] = env.loader.get_source(env, content[i])[0]
                            except Exception as e:  
                                self.log.exception(e)
        self.prepare_template()
    @property
    def upstream_list(self) -> List["BaseOperator"]: