**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/www/views.py
'unixname',
        'priority_weight',
        'queue',
        'queued_dttm',
        'try_number',
        'pool',
        'log_url',
    ]
    order_columns = [item for item in list_columns if item not in ['try_number', 'log_url']]
    search_columns = [
        'state',
        'dag_id',
        'task_id',
        'execution_date',
        'hostname',
        'queue',
        'pool',
        'operator',
        'start_date',
        'end_date',
    ]
    base_order = ('job_id', 'asc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    def log_url_formatter(self):
        """Formats log URL."""
        log_url = self.get('log_url')  
        return Markup(  
            '<a href="{log_url}"><span class="material-icons" aria-hidden="true">reorder</span></a>'
        ).format(log_url=log_url)
    def duration_f(self):