ActiveRecord changes return nilable types

[Morriar]

All the ActiveRecord methods used to access the previous change state of a column can actually return nil.

For example:

class Foo < ApplicationRecord; end

foo = Foo.new(name: "foo")
foo.name_previous_change # => nil

This pull-request uses T.nilable as return type for all the impacted methods.

[paracycle]

Good catch, thanks!