Fix capping quantity to stock levels by always validating line items with quantity zero

Stock::Quantifier.can_supply? returns false for an input of zero when stock level is negative

app/models/spree/stock/availability_validator_decorator.rb

@@ -4,6 +4,7 @@ Spree::Stock::AvailabilityValidator.class_eval do
     return if line_item.skip_stock_check
 
     quantity = adapt_line_item_quantity_to_inventory_units(line_item)
+    return if quantity == 0
 
     validate_quantity(line_item, quantity)
   end