**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
"""Retrieves Run."""
        dag_id = request.form.get('dag_id')
        task_id = request.form.get('task_id')
        origin = get_safe_url(request.form.get('origin'))
        dag = current_app.dag_bag.get_dag(dag_id)
        task = dag.get_task(task_id)
        execution_date = request.form.get('execution_date')
        execution_date = timezone.parse(execution_date)
        ignore_all_deps = request.form.get('ignore_all_deps') == "true"
        ignore_task_deps = request.form.get('ignore_task_deps') == "true"
        ignore_ti_state = request.form.get('ignore_ti_state') == "true"
        executor = ExecutorLoader.get_default_executor()
        valid_celery_config = False
        valid_kubernetes_config = False
        try:
            from airflow.executors.celery_executor import CeleryExecutor  
            valid_celery_config = isinstance(executor, CeleryExecutor)
        except ImportError:
            pass
        try:
            from airflow.executors.kubernetes_executor import KubernetesExecutor  
            valid_kubernetes_config = isinstance(executor, KubernetesExecutor)
        except ImportError:
            pass
        if not valid_celery_config and not valid_kubernetes_config:
            flash("Only works with the Celery or Kubernetes executors, sorry", "error")
            return redirect(origin)
        ti = models.TaskInstance(task=task, execution_date=execution_date)
        ti.refresh_from_db()