Use Null Promotion Configuration in core

We want to use the legacy `Spree::PromotionConfiguration` in the
solidus_legacy_promotions test suite.

core/lib/spree/app_configuration.rb

@@ -594,7 +594,7 @@ def stock
     #   the standard promotion configuration class Spree::Core::PromotionConfiguration.
     attr_writer :promotions
     def promotions
-      @promotions ||= Spree::Core::PromotionConfiguration.new
+      @promotions ||= Spree::Core::NullPromotionConfiguration.new
     end
 
     class << self

core/spec/lib/spree/app_configuration_spec.rb

@@ -41,7 +41,7 @@
   end
 
   it "uses core's promotion configuration class by default" do
-    expect(prefs.promotions).to be_a Spree::Core::PromotionConfiguration
+    expect(prefs.promotions).to be_a Spree::Core::NullPromotionConfiguration
   end
 
   context "deprecated preferences" do
@@ -52,23 +52,23 @@
     end
 
     it "uses order adjustments recalculator class by default" do
-      expect(prefs.promotion_adjuster_class).to eq Spree::Promotion::OrderAdjustmentsRecalculator
+      expect(prefs.promotion_adjuster_class).to eq Spree::NullPromotionAdjuster
     end
 
     it "uses promotion handler coupon class by default" do
-      expect(prefs.coupon_code_handler_class).to eq Spree::PromotionHandler::Coupon
+      expect(prefs.coupon_code_handler_class).to eq Spree::NullPromotionHandler
     end
 
     it "uses promotion handler shipping class by default" do
-      expect(prefs.shipping_promotion_handler_class).to eq Spree::PromotionHandler::Shipping
+      expect(prefs.shipping_promotion_handler_class).to eq Spree::NullPromotionHandler
     end
 
     it "uses promotion code batch mailer class by default" do
-      expect(prefs.promotion_code_batch_mailer_class).to eq Spree::PromotionCodeBatchMailer
+      expect(prefs.promotion_code_batch_mailer_class).to eq Spree::DeprecatedConfigurableClass
     end
 
     it "uses promotion chooser class by default" do
-      expect(prefs.promotion_chooser_class).to eq Spree::PromotionChooser
+      expect(prefs.promotion_chooser_class).to eq Spree::DeprecatedConfigurableClass
     end
   end
 
@@ -113,7 +113,7 @@
 
   describe '#promotions' do
     subject { prefs.promotions }
-    it { is_expected.to be_a Spree::Core::PromotionConfiguration }
+    it { is_expected.to be_a Spree::Core::NullPromotionConfiguration }
   end
 
   describe '@default_country_iso_code' do