Small fixes based on rbs research

[klaustopher]

While working on #12916 the type checker has identified those 2 issues. Regardless of if the PR gets merged, those fixes should be implemented.

# Current dev
[1] pry(main)> WorkPackageCustomField.first.work_packages
  WorkPackageCustomField Load (2.6ms)  SELECT "custom_fields".* FROM "custom_fields" WHERE "custom_fields"."type" = $1 ORDER BY "custom_fields"."id" ASC LIMIT $2  [["type", "WorkPackageCustomField"], ["LIMIT", 1]]
ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :work_package_custom_values in model WorkPackageCustomField
from /Users/klaustopher/.rvm/gems/ruby-3.2.1/gems/activerecord-7.0.5/lib/active_record/reflection.rb:944:in `check_validity!'

# This branch
[1] pry(main)> WorkPackageCustomField.first.work_packages
  WorkPackageCustomField Load (0.7ms)  SELECT "custom_fields".* FROM "custom_fields" WHERE "custom_fields"."type" = $1 ORDER BY "custom_fields"."id" ASC LIMIT $2  [["type", "WorkPackageCustomField"], ["LIMIT", 1]]
  WorkPackage Load (13.2ms)  SELECT "work_packages".* FROM "work_packages" INNER JOIN "custom_values" ON "work_packages"."id" = "custom_values"."customized_id" WHERE "custom_values"."custom_field_id" = $1 AND "custom_values"."customized_type" = $2  [["custom_field_id", 1], ["customized_type", "WorkPackage"]]
=> [#<WorkPackage:0x000000010dd2a128 ...>]