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/models/serialized_dag.py
"""Checks a DAG exist in serialized_dag table.
        :param dag_id: the DAG to check
        :param session: ORM Session
        """
        return session.query(literal(True)).filter(cls.dag_id == dag_id).first() is not None
    @classmethod
    @provide_session
    def get(cls, dag_id: str, session: Session = None) -> Optional['SerializedDagModel']:
        """
        Get the SerializedDAG for the given dag ID.
        It will cope with being passed the ID of a subdag by looking up the
        root dag_id from the DAG table.
        :param dag_id: the DAG to fetch
        :param session: ORM Session
        """
        row = session.query(cls).filter(cls.dag_id == dag_id).one_or_none()
        if row:
            return row
        
        
        root_dag_id = session.query(DagModel.root_dag_id).filter(DagModel.dag_id == dag_id).scalar()
        return session.query(cls).filter(cls.dag_id == root_dag_id).one_or_none()
    @staticmethod
    @provide_session
    def bulk_sync_to_db(dags: List[DAG], session: Session = None):