Can't test enums which aren't backed by a column

[Kutomore]

Due to how the code on https://github.com/thoughtbot/shoulda-matchers/blob/c934d7534520d90cf9d17f0cd1745bf95d38cf41/lib/shoulda/matchers/active_record/define_enum_for_matcher.rb#L234 is implemented, if you have a enum which is not backed by a column in your database you'll get an error when trying to run the test.

I've ran into this when creating a enum to match a field I added to my query through SQL only, which in teory is not saved on my database.

To reproduce simply create a enum that has no field on the database with the same name, and you'll get the error.

# frozen_string_literal: true

class Car < ApplicationRecord
  enum label: { perfect_match: 0, good_match: 1 }
end

describe Car do
  it { is_expected.to define_enum_for :label }
end

  1) Car is expected to define :label as an enum backed by an integer
     Failure/Error: it { is_expected.to define_enum_for :label }

     NoMethodError:
       undefined method `type' for nil:NilClass
     # /usr/local/bundle/gems/shoulda-matchers-5.0.0/lib/shoulda/matchers/active_record/define_enum_for_matcher.rb:363:in `column_type_matches?'
     # /usr/local/bundle/gems/shoulda-matchers-5.0.0/lib/shoulda/matchers/active_record/define_enum_for_matcher.rb:234:in `matches?'
     # ./spec/models/car_spec.rb:3:in `block (2 levels) in <main>'

I personally believew the type of the enum should only be checked if backed_by_column_of_type is called.