Fix shipping rate discount factory

This factory would violate a not-null constraint.

friendly_promotions/lib/solidus_friendly_promotions/testing_support/factories/friendly_shipping_rate_discount_factory.rb

@@ -4,7 +4,11 @@
   factory :friendly_shipping_rate_discount, class: "SolidusFriendlyPromotions::ShippingRateDiscount" do
     amount { BigDecimal("-4.00") }
     shipping_rate
-    promotion_action { SolidusFriendlyPromotions::Actions::AdjustShipment.new }
+    promotion_action do
+      promotion = create(:friendly_promotion, name: "10% off shipping!", customer_label: "10% off")
+      ten_percent = SolidusFriendlyPromotions::Calculators::Percent.new(preferred_percent: 10)
+      SolidusFriendlyPromotions::Actions::AdjustShipment.create!(promotion: promotion, calculator: ten_percent)
+    end
     label { "10% off" }
   end
 end

friendly_promotions/spec/lib/solidus_friendly_promotions/testing_support/factories_spec.rb

@@ -10,7 +10,8 @@
       :friendly_promotion_with_first_order_rule,
       :friendly_promotion_with_item_adjustment,
       :friendly_promotion_with_item_total_rule,
-      :friendly_promotion_with_order_adjustment
+      :friendly_promotion_with_order_adjustment,
+      :friendly_shipping_rate_discount
     ].each do |factory|
       expect { FactoryBot.create(factory) }.not_to raise_exception
     end