**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
"""Refresh everything"""
        if settings.STORE_SERIALIZED_DAGS:
            current_app.dag_bag.collect_dags_from_db()
        else:
            current_app.dag_bag.collect_dags(only_if_updated=False)
        
        for dag_id, dag in current_app.dag_bag.dags.items():
            current_app.appbuilder.sm.sync_perm_for_dag(dag_id, dag.access_control)
        flash("All DAGs are now up to date")
        return redirect(url_for('Airflow.index'))
    @expose('/gantt')
    @has_dag_access(can_dag_read=True)
    @has_access
    @action_logging
    @provide_session
    def gantt(self, session=None):
        """Show GANTT chart."""
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        demo_mode = conf.getboolean('webserver', 'demo_mode')
        root = request.args.get('root')
        if root:
            dag = dag.sub_dag(
                task_regex=root,
                include_upstream=True,
                include_downstream=False)
        dt_nr_dr_data = get_date_time_num_runs_dag_runs_form_data(request, session, dag)
        dttm = dt_nr_dr_data['dttm']
        form = DateTimeWithNumRunsWithDagRunsForm(data=dt_nr_dr_data)