**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
"""Dag statistics."""
        dr = models.DagRun
        allowed_dag_ids = current_app.appbuilder.sm.get_accessible_dag_ids()
        if 'all_dags' in allowed_dag_ids:
            allowed_dag_ids = [dag_id for dag_id, in session.query(models.DagModel.dag_id)]
        dag_state_stats = session.query(dr.dag_id, dr.state, sqla.func.count(dr.state))\
            .group_by(dr.dag_id, dr.state)
        
        selected_dag_ids = {
            unquote(dag_id) for dag_id in request.form.getlist('dag_ids') if dag_id
        }
        if selected_dag_ids:
            filter_dag_ids = selected_dag_ids.intersection(allowed_dag_ids)
        else:
            filter_dag_ids = allowed_dag_ids
        if not filter_dag_ids: