Prevent hard-errors when VatPriceGenerator is invoked without a defau…

…lt price

core/app/models/spree/price.rb

@@ -46,6 +46,8 @@ def price=(price)
     end
 
     def net_amount
+      return nil unless amount
+
       amount / (1 + sum_of_vat_amounts)
     end
 

core/app/models/spree/variant/vat_price_generator.rb

@@ -25,6 +25,7 @@ def initialize(variant)
       def run
         # Early return if there is no VAT rates in the current store.
         return if !variant.tax_category || variant_vat_rates.empty?
+        return unless variant.default_price.net_amount
 
         country_isos_requiring_price.each do |country_iso|
           # Don't re-create the default price