Serialise tag_list only for variant overrides

Don't try to call tag_list on just variants since that will fail.

Normally, all variants of `current_order` should be extended to
VariantOverrides of the current order cycle. But in development
environment, it can happen that the variants are reloaded without being
extended again.

app/serializers/api/variant_serializer.rb

@@ -1,7 +1,6 @@
 class Api::VariantSerializer < ActiveModel::Serializer
   attributes :id, :is_master, :count_on_hand, :name_to_display, :unit_to_display
   attributes :options_text, :on_demand, :price, :fees, :price_with_fees, :product_name
-  attributes :tag_list
 
   def price
     object.price
@@ -23,4 +22,8 @@ class Api::VariantSerializer < ActiveModel::Serializer
   def product_name
     object.product.name
   end
+
+  def tag_list
+    object.tag_list if object.is_a? VariantOverride
+  end
 end