**Apache Airflow version**:

master

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**: breeze

**What happened**:

This DAG
```python
from airflow import models
from airflow.operators.dummy_operator import DummyOperator
from airflow.utils.dates import days_ago

with models.DAG("test", start_date=days_ago(1), schedule_interval=None,) as dag:
    t1 = DummyOperator(task_id="t1")
    t2 = DummyOperator(task_id="t2")
    t1 >> t2
```

is rendering like that:

<img width="1374" alt="Screenshot 2020-08-27 at 19 59 41" src="https://user-images.githubusercontent.com/9528307/91478403-11d7fb00-e8a0-11ea-91d0-d7d578bcb5a2.png">

**What you expected to happen**:

I expect to see same relations as defined in DAG file

**How to reproduce it**:

Render the example DAG from above.


**Anything else we need to know**:

I'm surprised by this bug 👀 

airflow/www/views.py
"""Toggle paused."""
        dag_id = request.args.get('dag_id')
        is_paused = request.args.get('is_paused') == 'false'
        models.DagModel.get_dagmodel(dag_id).set_is_paused(
            is_paused=is_paused)
        return "OK"
    @expose('/refresh', methods=['POST'])
    @has_dag_access(can_dag_edit=True)
    @has_access
    @action_logging
    @provide_session
    def refresh(self, session=None):
        """Refresh DAG."""
        dag_id = request.values.get('dag_id')
        orm_dag = session.query(
            DagModel).filter(DagModel.dag_id == dag_id).first()
        if orm_dag:
            orm_dag.last_expired = timezone.utcnow()
            session.merge(orm_dag)
        session.commit()
        dag = current_app.dag_bag.get_dag(dag_id)
        
        current_app.appbuilder.sm.sync_perm_for_dag(dag_id, dag.access_control)
        flash("DAG [{}] is now fresh as a daisy".format(dag_id))
        return redirect(request.referrer)
    @expose('/refresh_all', methods=['POST'])
    @has_access
    @action_logging
    def refresh_all(self):