Fix typing for the getter of serialized columns

[wpolicarpo]

Motivation

According to this table, serialized columns always return an empty Array or Hash if their coders are either an Array or Hash respectively and the value in the database is null.

Currently tapioca assumes the getter will always return a nilable type, which is not true:

Toggle me for an example!

require "bundler/inline"

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

  gem "activerecord", "7.0.4.2"
  gem "sqlite3"
end

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

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :books, force: true do |t|
    t.string :authors
  end
end

class Book < ActiveRecord::Base
  serialize :authors, Array
end

class BookTest < Minitest::Test
  def test_populated_array
    book = Book.new(authors: ["foo", "bar"])
    assert_equal(["foo", "bar"], book.authors)
  end

  def test_empty_array
    book = Book.new(authors: [])
    assert_equal([], book.authors)
  end

  def test_nil
    book = Book.new(authors: nil)
    assert_equal([], book.authors)
  end
end

Implementation

This PR changes the getters return type for serialized columns where the coder is one of those exceptional classes.

Tests

Added tests for all getters, making sure the Array and Hash ones check for a not nilable type being returned.