TypeError: OAuth2Manager::getScopes(): Return value must be a string

I am more in favor of declaring that it can be either NULL or string.

getScopes as I see is called upon like this

    $extra_scopes = $this->getScopes();
    if ($extra_scopes) {
      $scopes = array_merge($scopes, explode(',', $extra_scopes));
    }

where it's better that we return either NULL or correct string

Assigning to myself for review.

I reviewed the MR's code, and it looks good to me.
Moving the issue to RTBC. Thanks!