oj: Oj v3.13 seems to be freezing empty string values

Hey! We recently updated Oj from version 3.12.3 to v3.13.1, and something changed (I guess in Oj.load?) and it is causing errors in our rails app.

Here is a reproducible test case:

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "activerecord", "~> 6.1.0"
  gem "sqlite3"
  gem "oj"
end

require "active_record"
require "minitest/autorun"
require "logger"
require "oj"

Oj.optimize_rails

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.json :profile
  end
end

class User < ActiveRecord::Base
  before_validation :strip_profile_fields

  def strip_profile_fields
    profile.each_value { |v| v.strip! if v.respond_to?(:strip!) }
  end
end

class BugTest < Minitest::Test
  def test_saving_with_empty_string
    User.create!(profile: { special_property: "", working_property: "hello  " })

    assert_equal 1, User.count
  end
end

Commenting Oj.optimize_rails will make the test pass. I lacks the Oj internal knowledge to know what could have changed.