**Apache Airflow version**:
1.10.10

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

kubeenetes ver: 1.15.5

**Environment**:

- **Cloud provider or hardware configuration**: Used docker based Airflow Image
- **OS** (e.g. from /etc/os-release): Debian
- **Kernel** (e.g. `uname -a`): 4.19.76-linuxkit x86_64 Linux


**What happened**:

When user navigates to tree view of dag and keep on clicking on `Go` button at `number of runs` the` base date` cycle back to one dag run and in the end when there is no dag run left, it shows `invalid date`. In the end user gets below exception: 

```
                          ____/ (  (    )   )  \___
                         /( (  (  )   _    ))  )   )\
                       ((     (   )(    )  )   (   )  )
                     ((/  ( _(   )   (   _) ) (  () )  )
                    ( (  ( (_)   ((    (   )  .((_ ) .  )_
                   ( (  )    (      (  )    )   ) . ) (   )
                  (  (   (  (   ) (  _  ( _) ).  ) . ) ) ( )
                  ( (  (   ) (  )   (  ))     ) _)(   )  )  )
                 ( (  ( \ ) (    (_  ( ) ( )  )   ) )  )) ( )
                  (  (   (  (   (_ ( ) ( _    )  ) (  )  )   )
                 ( (  ( (  (  )     (_  )  ) )  _)   ) _( ( )
                  ((  (   )(    (     _    )   _) _(_ (  (_ )
                   (_((__(_(__(( ( ( |  ) ) ) )_))__))_)___)
                   ((__)        \\||lll|l||///          \_))
                            (   /(/ (  )  ) )\   )
                          (    ( ( ( | | ) ) )\   )
                           (   /(| / ( )) ) ) )) )
                         (     ( ((((_(|)_)))))     )
                          (      ||\(|(|)|/||     )
                        (        |(||(||)||||        )
                          (     //|/l|||)|\\ \     )
                        (/ / //  /|//||||\\  \ \  \ _)
-------------------------------------------------------------------------------
Node: c6059861e1e9
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 352, in _parse
    yearfirst=self._options['year_first']
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/_parser.py", line 1374, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/_parser.py", line 649, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: Invalid date
During handling of the above exception, another exception occurred:
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_rbac/decorators.py", line 121, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 92, in view_func
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 56, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/views.py", line 1406, in tree
    base_date = timezone.parse(base_date)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/timezone.py", line 180, in parse
    return pendulum.parse(string, tz=timezone or TIMEZONE)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parser.py", line 75, in parse
    return Parser(**options).parse(text)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parser.py", line 31, in parse
    parsed = super(Parser, self).parse(text)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 297, in parse
    return self.normalize(self._parse(text))
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 355, in _parse
    raise ParserError('Invalid date string: {}'.format(text))
pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date
```

**What you expected to happen**:

User should not receive exception pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date 

**How to reproduce it**:

1. Create a sample dag/example dag with shorter scheduled interval 
2. Have 2-3 dag runs.
3. Navigate to tree view and click on `Go `button
4. Keep clicking on Go button till there are no dag runs remaining, you would get  `pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date `exception.

**Screenshot**:

![image](https://user-images.githubusercontent.com/60378152/93097661-78904d80-f6c3-11ea-8616-bf7ac5b17293.png)

![image](https://user-images.githubusercontent.com/60378152/93097682-81811f00-f6c3-11ea-941d-0e82e71f198a.png)

![image](https://user-images.githubusercontent.com/60378152/93097729-8e9e0e00-f6c3-11ea-884c-8ec6ba363fa4.png)


How often does this problem occur? Once? Every time etc?

Everytime


airflow/models/dag.py
"""
    Returns the last dag run for a dag, None if there was none.
    Last dag run can be any type of run eg. scheduled or backfilled.
    Overridden DagRuns are ignored.
    """
    DR = DagRun
    query = session.query(DR).filter(DR.dag_id == dag_id)
    if not include_externally_triggered:
        query = query.filter(DR.external_trigger == False)  
    query = query.order_by(DR.execution_date.desc())
    return query.first()
@functools.total_ordering
class DAG(LoggingMixin):
    """
    A dag (directed acyclic graph) is a collection of tasks with directional
    dependencies. A dag also has a schedule, a start date and an end date
    (optional). For each schedule, (say daily or hourly), the DAG needs to run
    each individual tasks as their dependencies are met. Certain tasks have
    the property of depending on their own past, meaning that they can't run
    until their previous schedule (and upstream tasks) are completed.
    DAGs essentially act as namespaces for tasks. A task_id can only be
    added once to a DAG.
    :param dag_id: The id of the DAG; must consist exclusively of alphanumeric
        characters, dashes, dots and underscores (all ASCII)
    :type dag_id: str
    :param description: The description for the DAG to e.g. be shown on the webserver