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
"""Deletes a DAG with given dag_id.
        :param dag_id: dag_id to be deleted
        :param session: ORM Session
        """
        
        session.execute(cls.__table__.delete().where(cls.dag_id == dag_id))
    @classmethod
    @provide_session
    def remove_deleted_dags(cls, alive_dag_filelocs: List[str], session=None):
        """Deletes DAGs not included in alive_dag_filelocs.
        :param alive_dag_filelocs: file paths of alive DAGs
        :param session: ORM Session
        """
        alive_fileloc_hashes = [DagCode.dag_fileloc_hash(fileloc) for fileloc in alive_dag_filelocs]
        log.debug(
            "Deleting Serialized DAGs (for which DAG files are deleted) from %s table ", cls.__tablename__
        )
        
        session.execute(
            cls.__table__.delete().where(
                and_(cls.fileloc_hash.notin_(alive_fileloc_hashes), cls.fileloc.notin_(alive_dag_filelocs))
            )
        )
    @classmethod
    @provide_session
    def has_dag(cls, dag_id: str, session: Session = None) -> bool: