**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
"""Triggers DAG Run."""
        dag_id = request.values.get('dag_id')
        origin = get_safe_url(request.values.get('origin'))
        if request.method == 'GET':
            return self.render_template(
                'airflow/trigger.html',
                dag_id=dag_id,
                origin=origin,
                conf=''
            )
        dag_orm = session.query(models.DagModel).filter(models.DagModel.dag_id == dag_id).first()
        if not dag_orm:
            flash("Cannot find dag {}".format(dag_id))
            return redirect(origin)
        execution_date = timezone.utcnow()
        dr = DagRun.find(dag_id=dag_id, execution_date=execution_date, run_type=DagRunType.MANUAL)
        if dr:
            flash(f"This run_id {dr.run_id} already exists")  
            return redirect(origin)
        run_conf = {}
        request_conf = request.values.get('conf')
        if request_conf:
            try:
                run_conf = json.loads(request_conf)