Allow unsetting "Required pipeline configuration" for CI/CD

After setting "required pipeline configuration", users should be able to
set it to "no required pipeline". They could not do this because
we send required_instance_ci_template empty string for that value.

With this commit, we started to behave empty string value of this
attribute as nil value.

app/controllers/admin/application_settings_controller.rb

@@ -192,6 +192,7 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
     params[:application_setting][:import_sources]&.delete("")
     params[:application_setting][:restricted_visibility_levels]&.delete("")
     params[:application_setting].delete(:elasticsearch_aws_secret_access_key) if params[:application_setting][:elasticsearch_aws_secret_access_key].blank?
+    params[:application_setting][:required_instance_ci_template] = nil if params[:application_setting][:required_instance_ci_template].blank?
     # TODO Remove domain_blacklist_raw in APIv5 (See https://gitlab.com/gitlab-org/gitlab-foss/issues/67204)
     params.delete(:domain_blacklist_raw) if params[:domain_blacklist_file]
     params.delete(:domain_blacklist_raw) if params[:domain_blacklist]

ee/changelogs/unreleased/13264-allow-to-set-required_instance_ci_template-as-nil.yml

+---
+title: Allow unsetting "Required pipeline configuration" for CI/CD
+merge_request: 28432
+author:
+type: fixed

ee/spec/controllers/admin/application_settings_controller_spec.rb

@@ -153,6 +153,35 @@ describe Admin::ApplicationSettingsController do
       it_behaves_like 'settings for licensed features'
     end
 
+    context 'required instance ci template' do
+      let(:settings) { { required_instance_ci_template: 'Auto-DevOps' } }
+      let(:feature) { :required_ci_templates }
+
+      it_behaves_like 'settings for licensed features'
+
+      context 'when ApplicationSetting already has a required_instance_ci_template value' do
+        before do
+          ApplicationSetting.current.update!(required_instance_ci_template: 'Auto-DevOps')
+        end
+
+        context 'with a valid value' do
+          let(:settings) { { required_instance_ci_template: 'Code-Quality' } }
+
+          it_behaves_like 'settings for licensed features'
+        end
+
+        context 'with an empty value' do
+          it 'sets required_instance_ci_template as nil' do
+            stub_licensed_features(required_ci_templates: true)
+
+            put :update, params: { application_setting: { required_instance_ci_template: '' } }
+
+            expect(ApplicationSetting.current.required_instance_ci_template).to be_nil
+          end
+        end
+      end
+    end
+
     it 'updates repository_size_limit' do
       put :update, params: { application_setting: { repository_size_limit: '100' } }