Requested changes on VariantOverrride

- change was not exactly equivalent, so reverting +
  rubo comment

app/models/variant_override.rb

@@ -52,7 +52,14 @@ class VariantOverride < ApplicationRecord
       return
     end
 
-    update!(count_on_hand: (count_on_hand || 0) + quantity)
+    # rubocop:disable Rails/SkipsModelValidations
+    # Cf. conversation https://github.com/openfoodfoundation/openfoodnetwork/pull/12647
+    if quantity > 0
+      increment! :count_on_hand, quantity
+    elsif quantity < 0
+      decrement! :count_on_hand, -quantity
+    end
+    # rubocop:enable Rails/SkipsModelValidations
   end
 
   def default_stock?