feat: use location.postcode rather than nil

spec/features/form/check_answers_page_lettings_logs_spec.rb

@@ -18,7 +18,7 @@ RSpec.describe "Lettings Log Check Answers Page" do
       needstype: 2,
       scheme:,
       location:,
-      postcode_full: nil,
+      postcode_full: location.postcode,
     )
   end
   let(:empty_lettings_log) do

spec/models/lettings_log_spec.rb

@@ -1965,8 +1965,8 @@ RSpec.describe LettingsLog do
       context "when there is a duplicate supported housing log" do
         let(:scheme) { create(:scheme, owning_organisation: organisation) }
         let(:location) { create(:location, scheme:) }
-        let!(:supported_housing_log) { create(:lettings_log, :duplicate, needstype: 2, location:, scheme:, owning_organisation: organisation, postcode_full: nil) }
-        let!(:duplicate_supported_housing_log) { create(:lettings_log, :duplicate, needstype: 2, location:, scheme:, owning_organisation: organisation, postcode_full: nil) }
+        let!(:supported_housing_log) { create(:lettings_log, :duplicate, needstype: 2, location:, scheme:, owning_organisation: organisation, postcode_full: location.postcode) }
+        let!(:duplicate_supported_housing_log) { create(:lettings_log, :duplicate, needstype: 2, location:, scheme:, owning_organisation: organisation, postcode_full: location.postcode) }
 
         it "returns the log as a duplicate" do
           expect(duplicate_sets).to contain_exactly(