**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
'task_id': wwwutils.task_instance_link,
        'start_date': wwwutils.datetime_f('start_date'),
        'end_date': wwwutils.datetime_f('end_date'),
        'execution_date': wwwutils.datetime_f('execution_date'),
        'reschedule_date': wwwutils.datetime_f('reschedule_date'),
        'duration': duration_f,
    }
class TaskInstanceModelView(AirflowModelView):
    """View to show records from TaskInstance table"""
    route_base = '/taskinstance'
    datamodel = AirflowModelView.CustomSQLAInterface(models.TaskInstance)  
    base_permissions = ['can_list']
    page_size = PAGE_SIZE
    list_columns = ['state', 'dag_id', 'task_id', 'execution_date', 'operator',
                    'start_date', 'end_date', 'duration', 'job_id', 'hostname',
                    'unixname', 'priority_weight', 'queue', 'queued_dttm', 'try_number',
                    'pool', 'log_url']
    order_columns = [item for item in list_columns if item not in ['try_number', 'log_url']]
    search_columns = ['state', 'dag_id', 'task_id', 'execution_date', 'hostname',
                      'queue', 'pool', 'operator', 'start_date', 'end_date']
    base_order = ('job_id', 'asc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    def log_url_formatter(self):
        """Formats log URL."""
        log_url = self.get('log_url')  
        return Markup(  
            '<a href="{log_url}">'
            '    <span class="glyphicon glyphicon-book" aria-hidden="true">'
            '</span></a>').format(log_url=log_url)
    def duration_f(self):