fix: check None before returning config

[mumarkhan999]

Description
While replacing python-memcache with pymemcache ticket link we got the following error

  File "/edx/app/edxapp/edx-platform/xmodule/video_block/video_block.py", line 244, in student_view
    fragment = Fragment(self.get_html())
  File "/edx/app/edxapp/edx-platform/xmodule/video_block/video_block.py", line 355, in get_html
    branding_info = BrandingInfoConfig.get_config().get(user_location)
  File "/edx/app/edxapp/edx-platform/lms/djangoapps/branding/models.py", line 54, in get_config
    return json.loads(info.configuration) if info.enabled else {}
AttributeError: 'NoneType' object has no attribute 'enabled'

The reason of above mentioned error was: django-config-models was returning None.

Which was then used by edx-platform which tries to retrieve values from None

https://github.com/openedx/edx-platform/blob/master/lms/djangoapps/branding/models.py#L54

This causes errors. In this PR, we're making sure that ConfigurationModel.current doesn't not return None.

[mumarkhan999]

[inform] The None check was there before at this place. But it was removed when TieredCache was included.

https://github.com/openedx/django-config-models/pull/55/files#diff-89972c71903f4383220fc6ece46180b9c5c52f3467b07de6cc662ae9a546f49aL199-L201