**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
include_downstream=False)
        arrange = request.args.get('arrange', dag.orientation)
        nodes = []
        edges = []
        for dag_task in dag.tasks:
            nodes.append({
                'id': dag_task.task_id,
                'value': {
                    'label': dag_task.task_id,
                    'labelStyle': "fill:{0};".format(dag_task.ui_fgcolor),
                    'style': "fill:{0};".format(dag_task.ui_color),
                    'rx': 5,
                    'ry': 5,
                }
            })
        def get_downstream(task):
            for downstream_task in task.downstream_list:
                edge = {
                    'source_id': downstream_task.task_id,
                    'target_id': downstream_task.task_id,
                }
                if edge not in edges:
                    edges.append(edge)
                    get_downstream(downstream_task)
        for dag_task in dag.roots:
            get_downstream(dag_task)
        dt_nr_dr_data = get_date_time_num_runs_dag_runs_form_data(request, session, dag)
        dt_nr_dr_data['arrange'] = arrange
        dttm = dt_nr_dr_data['dttm']
        class GraphForm(DateTimeWithNumRunsWithDagRunsForm):