[#53704] BUG Visible=false project attribute values are deleted when a non-admin user edits the attributes (2/2)

[dombesz]

Part 2 of the https://community.openproject.org/wp/53704

Fixing the pending spec: https://github.com/opf/openproject/pull/15296/files#diff-fe00deb197cdde8955e039ce8b85ba36c57733622ad8753f4553379d57cc0e86R465-R471

Description

The callback Projects::ActsAsCustomizablePatches#disable_custom_fields_with_empty_values should be an after_save callback, because the custom_values association has autosave enabled by default and it has an after_create callback in the model (CustomValue#activate_custom_field_in_customized_project).

The after_create callback in the children objects are ran after the after_create callbacks on the parent. In order to make sure we execute this callback after the children's callbacks, the after_save hook must be used.