**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
list_columns = ['id', 'dttm', 'dag_id', 'task_id', 'event', 'execution_date',
                    'owner', 'extra']
    search_columns = ['dag_id', 'task_id', 'event', 'execution_date', 'owner', 'extra']
    base_order = ('dttm', 'desc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    formatters_columns = {
        'dttm': wwwutils.datetime_f('dttm'),
        'execution_date': wwwutils.datetime_f('execution_date'),
        'dag_id': wwwutils.dag_link,
    }
class TaskRescheduleModelView(AirflowModelView):
    """View to show records from Task Reschedule table"""
    route_base = '/taskreschedule'
    datamodel = AirflowModelView.CustomSQLAInterface(models.TaskReschedule)  
    base_permissions = ['can_list']
    list_columns = ['id', 'dag_id', 'task_id', 'execution_date', 'try_number',
                    'start_date', 'end_date', 'duration', 'reschedule_date']
    search_columns = ['dag_id', 'task_id', 'execution_date', 'start_date', 'end_date',
                      'reschedule_date']
    base_order = ('id', 'desc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    def duration_f(self):
        """Duration calculation."""
        end_date = self.get('end_date')  
        duration = self.get('duration')  
        if end_date and duration:
            return timedelta(seconds=duration)
        return None
    formatters_columns = {
        'dag_id': wwwutils.dag_link,