bugfix: error_data statement when null

[crazyministr]

I've got an error during request to create adset. The error reason doesn't match with pull request changes but there is another problem.

1. 'error_data': 'null',
2. json.loads('null') returns NoneType object
3. error_data.get('blame_field_specs') throws an exception AttributeError: 'NoneType' object has no attribute 'get'

   {
   'message': 'Invalid parameter', 
   'type': 'OAuthException', 
   'code': 100, 
   'error_data': 'null', 
   'error_subcode': 1487079, 
   'is_transient': False, 
   'error_user_title': 'Invalid targeting spec', 
   'error_user_msg': 'The specified targeting spec is not valid because: The field audience_positions is not a valid target spec field', 
   'fbtrace_id': 'Ai7361tVWYjx2dvScd15MG9'
   }