FIX: Always include custom fields in CategorySerializer

even if it is empty

spec/serializers/category_serializer_spec.rb

@@ -5,6 +5,9 @@ describe CategorySerializer do
   let(:category) { Fabricate(:category) }
 
   it "includes custom fields" do
+    json = described_class.new(category, scope: Guardian.new, root: false).as_json
+    expect(json[:custom_fields]).to be_empty
+
     category.custom_fields["enable_marketplace"] = true
     category.save_custom_fields