I have a question about `ActiveRecord.locking_column`.

[UG Muscle]

Suppose the User model defines a column like this and does not have lock_version

```

class DeviseCreateUsers < ActiveRecord::Migration[5.1]

   def change

     create_table :users do |t|

       t.string :name

       # do not have a lock_version and other optimistic locks

     end

   end

end

```

When executing User.locking_column, I thought that I would return nil because This Model does not have locking_column, but return "locking_column".

Would you like to submit a PR that returns nil if there is no problem?

Thanks to read🙏