**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
node[children_key] = children
            if task.depends_on_past:
                node['depends_on_past'] = task.depends_on_past
            if task.start_date:
                
                node['start_ts'] = int(task.start_date.timestamp())
                if task.end_date:
                    
                    node['end_ts'] = int(task.end_date.timestamp())
            if task.extra_links:
                node['extra_links'] = task.extra_links
            return node
        data = {
            'name': '[DAG]',
            'children': [recurse_nodes(t, set()) for t in dag.roots],
            'instances': [
                dag_runs.get(d) or {'execution_date': d.isoformat()}
                for d in dates
            ],
        }
        form = DateTimeWithNumRunsForm(data={'base_date': max_date,
                                             'num_runs': num_runs})
        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
        
        data = htmlsafe_json_dumps(data, separators=(',', ':'))