**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
"""Graph Form class."""
            arrange = SelectField("Layout", choices=(
                ('LR', "Left->Right"),
                ('RL', "Right->Left"),
                ('TB', "Top->Bottom"),
                ('BT', "Bottom->Top"),
            ))
        form = GraphForm(data=dt_nr_dr_data)
        form.execution_date.choices = dt_nr_dr_data['dr_choices']
        task_instances = {
            ti.task_id: alchemy_to_dict(ti)
            for ti in dag.get_task_instances(dttm, dttm)}
        tasks = {
            t.task_id: {
                'dag_id': t.dag_id,
                'task_type': t.task_type,
                'extra_links': t.extra_links,
            }
            for t in dag.tasks}
        if not tasks:
            flash("No tasks found", "error")
        session.commit()
        doc_md = wwwutils.wrapped_markdown(getattr(dag, 'doc_md', None), css_class='dag-doc')
        task_log_reader = TaskLogReader()
        if task_log_reader.supports_external_link:
            external_log_name = task_log_reader.log_handler.log_name
        else:
            external_log_name = None
        return self.render_template(
            'airflow/graph.html',