This issue can be reproduced by clicking "Edit" any connection whose `extra` is `NULL` in DB, for example, "airflow_db", or "databricks_default"

I have already found the reason, will raise a PR shortly.


1. How to Reproduce

![Airflow](https://user-images.githubusercontent.com/11539188/98437154-61c51080-20e0-11eb-9e59-a9a628fa1f41.png)

2. Error

```
Python version: 3.6.10
Airflow version: 2.0.0a2
Node: a1c5c0c0026a
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.6/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/flask_appbuilder/views.py", line 602, in edit
    widgets = self._edit(pk)
  File "/usr/local/lib/python3.6/site-packages/flask_appbuilder/baseviews.py", line 1228, in _edit
    self.prefill_form(form, pk)
  File "/usr/local/lib/python3.6/site-packages/airflow/www/views.py", line 2546, in prefill_form
    extra_dictionary = json.loads(form.data.get('extra', '{}'))
  File "/usr/local/lib/python3.6/json/__init__.py", line 348, in loads
    'not {!r}'.format(s.__class__.__name__))
TypeError: the JSON object must be str, bytes or bytearray, not 'NoneType'
```



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airflow/www/views.py
dag=dag,
            chart=Markup(chart.htmlcontent),
            height=str(chart_height + 100) + "px",
            demo_mode=conf.getboolean('webserver', 'demo_mode'),
            root=root,
            form=form,
            tab_title='Landing times',
        )
    @expose('/paused', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    def paused(self):
        """Toggle paused."""
        dag_id = request.args.get('dag_id')
        is_paused = request.args.get('is_paused') == 'false'
        models.DagModel.get_dagmodel(dag_id).set_is_paused(is_paused=is_paused)
        return "OK"
    @expose('/refresh', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    @provide_session
    def refresh(self, session=None):