FIX: New general category changes preventing topic create (#18459)

* FIX: New general category changes preventing topic create

Follow up to: #18383

The logic in the previous commit was checking for null, but we are
seeding the SiteSetting.general_category_id with an id of -1 so we need
to check for a positive value as well as checking for null.

See: https://meta.discourse.org/t/240661

* Add js test for presence of category… dropdown option

app/assets/javascripts/select-kit/addon/components/category-chooser.js

@@ -45,7 +45,7 @@ export default ComboBoxComponent.extend({
       return Category.findUncategorized();
     } else {
       const generalCategoryId = this.siteSettings.general_category_id;
-      if (!generalCategoryId) {
+      if (!generalCategoryId || generalCategoryId < 0) {
         return this.defaultItem(null, htmlSafe(I18n.t("category.choose")));
       }
     }