Handle nil restricted visibility settings

Return `true` from `non_restricted_level?` when the
`restricted_visibility_levels` setting is nil.

lib/gitlab/visibility_level.rb

@@ -35,8 +35,12 @@ module Gitlab
       end
 
       def non_restricted_level?(level)
+        if current_application_settings.restricted_visibility_levels.nil?
+          true
+        else
           ! current_application_settings.restricted_visibility_levels.include?(level)
         end
+      end
 
       def valid_level?(level)
         options.has_value?(level)