**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
@has_access
    @provide_session
    def code(self, session=None):
        """Dag Code."""
        all_errors = ""
        dag_orm = None
        dag_id = None
        try:
            dag_id = request.args.get('dag_id')
            dag_orm = DagModel.get_dagmodel(dag_id, session=session)
            code = DagCode.get_code_by_fileloc(dag_orm.fileloc)
            html_code = Markup(highlight(
                code, lexers.PythonLexer(), HtmlFormatter(linenos=True)))  
        except Exception as e:  
            all_errors += (
                "Exception encountered during " +
                "dag_id retrieval/dag retrieval fallback/code highlighting:\n\n{}\n".format(e)
            )
            html_code = Markup('<p>Failed to load file.</p><p>Details: {}</p>').format(  
                escape(all_errors))
        return self.render_template(
            'airflow/dag_code.html', html_code=html_code, dag=dag_orm, title=dag_id,
            root=request.args.get('root'),
            demo_mode=conf.getboolean('webserver', 'demo_mode'),
            wrapped=conf.getboolean('webserver', 'default_wrap'))
    @expose('/dag_details')
    @has_dag_access(can_dag_read=True)
    @has_access
    @provide_session
    def dag_details(self, session=None):