Handle null restricted_visibility_levels setting

Fix a 500 error when the `restricted_visibility_levels` setting is null
in the database.

app/helpers/visibility_level_helper.rb

@@ -62,6 +62,6 @@ def visibility_level_label(level)
 
   def restricted_visibility_levels(show_all = false)
     return [] if current_user.is_admin? && !show_all
-    current_application_settings.restricted_visibility_levels
+    current_application_settings.restricted_visibility_levels || []
   end
 end