Python version: 3.7.9

Airflow version: 2.1.0

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/decorators.py", line 97, in view_func
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/views.py", line 4003, in list
    if SerializedDagModel.get_max_last_updated_datetime() > self.last_refresh:
TypeError: '>' not supported between instances of 'NoneType' and 'datetime.datetime'

**What you expected to happen**:
See the dags dependencies

**What do you think went wrong?**
It's happen only if I don't have any dag yet.

**How to reproduce it**:
With any dag created click in 
menu -> browser -> dag-dependencies
<!---

airflow/www/views.py
"""Refresh DAG."""
        dag_id = request.values.get('dag_id')
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == dag_id).first()
        if orm_dag:
            orm_dag.last_expired = timezone.utcnow()
            session.merge(orm_dag)
        session.commit()
        dag = current_app.dag_bag.get_dag(dag_id)
        
        current_app.appbuilder.sm.sync_perm_for_dag(dag_id, dag.access_control)
        flash(f"DAG [{dag_id}] is now fresh as a daisy")
        return redirect(request.referrer)
    @expose('/refresh_all', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    def refresh_all(self):
        """Refresh everything"""
        current_app.dag_bag.collect_dags_from_db()
        
        for dag_id, dag in current_app.dag_bag.dags.items():
            current_app.appbuilder.sm.sync_perm_for_dag(dag_id, dag.access_control)
        flash("All DAGs are now up to date")
        return redirect(url_for('Airflow.index'))
    @expose('/gantt')
    @auth.has_access(
        [