**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
"""@property: extra links for the task"""
        return list(
            set(self.operator_extra_link_dict.keys()).union(self.global_operator_extra_link_dict.keys())
        )
    def get_extra_links(self, dttm: datetime, link_name: str) -> Optional[Dict[str, Any]]:
        """
        For an operator, gets the URL that the external links specified in
        `extra_links` should point to.
        :raise ValueError: The error message of a ValueError will be passed on through to
            the fronted to show up as a tooltip on the disabled link
        :param dttm: The datetime parsed execution date for the URL being searched for
        :param link_name: The name of the link we're looking for the URL for. Should be
            one of the options specified in `extra_links`
        :return: A URL
        """
        if link_name in self.operator_extra_link_dict:
            return self.operator_extra_link_dict[link_name].get_link(self, dttm)
        elif link_name in self.global_operator_extra_link_dict:
            return self.global_operator_extra_link_dict[link_name].get_link(self, dttm)
        else:
            return None
    @classmethod
    def get_serialized_fields(cls):