FIX: Reset related site settings on general category delete (#18548)

* FIX: Reset related site settings on general category delete

If the new seeded General category is deleted we also need to delete the
corresponding site setting for it so that we don't try and reference it.

This fixes a bug in the category dropdown composer.

This change creates the `clear_related_site_settings` after destroy
hook that could also be used by other features in the future, like maybe
when we have a `default_category_id` site_setting.

Looks like if `nil` out a site setting it is set to `0`?

```
[9] pry(main)> SiteSetting.general_category_id = nil
  SiteSetting Load (0.4ms)  SELECT "site_settings".* FROM "site_settings" WHERE "site_settings"."name" = 'general_category_id' LIMIT 1
=> nil
[10] pry(main)> SiteSetting.general_category_id
=> 0
```

That is why the tests check if the value is `< 1` and not `nil`.

* Use -1 instead of nil because it is the default

app/models/category.rb

@@ -70,6 +70,7 @@ class Category < ActiveRecord::Base
 
   after_create :create_category_definition
   after_destroy :trash_category_definition
+  after_destroy :clear_related_site_settings
 
   before_save :apply_permissions
   before_save :downcase_email
@@ -312,6 +313,12 @@ class Category < ActiveRecord::Base
     self.topic&.trash!
   end
 
+  def clear_related_site_settings
+    if self.id == SiteSetting.general_category_id
+      SiteSetting.general_category_id = -1
+    end
+  end
+
   def topic_url
     if has_attribute?("topic_slug")
       Topic.relative_url(topic_id, read_attribute(:topic_slug))

spec/models/category_spec.rb

@@ -1286,4 +1286,16 @@ RSpec.describe Category do
       end
     end
   end
+
+  describe '#deleting the general category' do
+    fab!(:category) { Fabricate(:category) }
+
+    it 'should empty out the general_category_id site_setting' do
+      SiteSetting.general_category_id = category.id
+      category.destroy
+
+      expect(SiteSetting.general_category_id).to_not eq(category.id)
+      expect(SiteSetting.general_category_id).to be < 1
+    end
+  end
 end