Convert empty DAST excluded_urls to nil

- Convert excluded_urls to nil when array is empty
- Add spec

ee/app/services/dast_on_demand_scans/params_create_service.rb

@@ -73,7 +73,7 @@ module DastOnDemandScans
       return {} unless dast_site_profile
 
       {
-        excluded_urls: dast_site_profile.excluded_urls.join(','),
+        excluded_urls: dast_site_profile.excluded_urls.presence&.join(','),
         auth_username_field: dast_site_profile.auth_username_field,
         auth_password_field: dast_site_profile.auth_password_field,
         auth_username: dast_site_profile.auth_username

ee/spec/services/dast_on_demand_scans/params_create_service_spec.rb

@@ -70,6 +70,14 @@ RSpec.describe DastOnDemandScans::ParamsCreateService do
           )
         end
 
+        context 'when dast_site_profile.excluded_urls is empty' do
+          let_it_be(:dast_site_profile) { create(:dast_site_profile, project: project, excluded_urls: []) }
+
+          it 'returns nil' do
+            expect(subject.payload[:excluded_urls]).to be_nil
+          end
+        end
+
         context 'when the target is not validated and an active scan is requested' do
           let_it_be(:active_dast_scanner_profile) { create(:dast_scanner_profile, project: project, scan_type: 'active') }