Fix memoization logic in Localized mixin (#465)

Right now, the memoization logic will return `nil` if `@localizations` was set to `nil`. This means we don't have a straightforward way to reset this class-level variable. It will be important in subsequent PRs to be able to do so when running tests.

If the localizations are empty, `@localizations` will anyway be set to `{}`, not `nil`. This PR ensures that the localizations are loaded instead of early returning when `@localizations == nil`.

dev/lib/product_taxonomy/models/mixins/localized.rb

@@ -3,7 +3,7 @@
 module ProductTaxonomy
   module Localized
     def localizations
-      return @localizations if defined?(@localizations)
+      return @localizations if @localizations
 
       humanized_name = name.split("::").last.humanize.downcase.pluralize
       localization_path = File.join(DATA_PATH, "localizations", humanized_name, "*.yml")