Implement required method

Most of the time this doesn't get called because source_required: false.
But sometimes it [does happen](https://app.bugsnag.com/yaycode/openfoodnetwork-uk/errors/66329690f4b6380007e8a4f8)

I have a feeling that source_required? could be moved to the superclass as payment_source_class.present?. But I don't know enough about this area of the system to try it...

app/models/spree/payment_method/check.rb

@@ -25,6 +25,10 @@ module Spree
         ActiveMerchant::Billing::Response.new(true, "", {}, {})
       end
 
+      def payment_source_class
+        nil
+      end
+
       def source_required?
         false
       end

spec/controllers/checkout_controller_spec.rb

@@ -338,12 +338,11 @@ RSpec.describe CheckoutController, type: :controller do
           let!(:payment_method_id) { payment_method.id }
 
           it "updates and redirects to summary step" do
-            expect { put(:update, params:) }.to raise_error(RuntimeError, error_message)
+            put(:update, params:)
 
-            # according to Bugsnag, we should get an error 500
-            expect(response.status).to be 200
-            expect(response).not_to redirect_to checkout_step_path(:summary)
-            expect(order.reload.state).to eq "payment"
+            expect(response.status).to be 302
+            expect(response).to redirect_to checkout_step_path(:summary)
+            expect(order.reload.state).to eq "confirmation"
           end
         end