**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
"""Formats duration."""
        end_date = self.get('end_date')  
        duration = self.get('duration')  
        if end_date and duration:
            return timedelta(seconds=duration)
        return None
    formatters_columns = {
        'log_url': log_url_formatter,
        'task_id': wwwutils.task_instance_link,
        'hostname': wwwutils.nobr_f('hostname'),
        'state': wwwutils.state_f,
        'execution_date': wwwutils.datetime_f('execution_date'),
        'start_date': wwwutils.datetime_f('start_date'),
        'end_date': wwwutils.datetime_f('end_date'),
        'queued_dttm': wwwutils.datetime_f('queued_dttm'),
        'dag_id': wwwutils.dag_link,
        'duration': duration_f,
    }
    @provide_session
    @action('clear', lazy_gettext('Clear'),
            lazy_gettext('Are you sure you want to clear the state of the selected task'
                         ' instance(s) and set their dagruns to the running state?'),
            single=False)
    def action_clear(self, task_instances, session=None):