Handle potentially null `project_feature` in `Project` entity

Description

Locally I see a 500 response when requesting http://gitlab.localhost:3001/api/v4/projects?archived=no&per_page=100.

Analysis

The development.log shows this error:

NoMethodError (undefined method `string_access_level' for nil:NilClass):
  /path/to/gdk-ee/gitlab/lib/api/entities.rb:276:in `block in <class:Project>'

I think there is an issue with these *_access_level properties not appropriately handling projects with null project_features.