Fix instance default option being true as value

When a user selected 'Instance default' as value for the auto devops
settings, this was interpreted as a true value. Now we post an empty
string in this case, meaning we want to set `NULL` in the database.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37893#note_40541294

app/views/projects/pipelines_settings/_show.html.haml

@@ -25,8 +25,8 @@
                 %span.descr
                   A specific .gitlab-ci.yml file needs to be specified before you can begin using Continious Integration and Delivery.
             .radio
-              = form.label :enabled do
-                = form.radio_button :enabled, nil
+              = form.label :enabled_nil do
+                = form.radio_button :enabled, ''
                 %strong
                   Instance default (status: #{current_application_settings.auto_devops_enabled?})
                 %br

spec/controllers/projects/pipelines_setttings_controller_spec.rb → spec/controllers/projects/pipelines_settings_controller_spec.rb

@@ -17,15 +17,27 @@ describe Projects::PipelinesSettingsController do
         namespace_id: project.namespace.to_param,
         project_id: project,
         project: {
-          auto_devops_attributes: { enabled: false, domain: 'mempmep.md' }
+          auto_devops_attributes: params
         }
     end
 
     context 'when updating the auto_devops settings' do
+      let(:params) { { enabled: '', domain: 'mepmep.md' } }
+
       it 'redirects to the settings page' do
         expect(response).to have_http_status(302)
         expect(flash[:notice]).to eq("Pipelines settings for '#{project.name}' were successfully updated.")
       end
+
+      context 'following the instance default' do
+        let(:params) { { enabled: '' } }
+
+        it 'allows enabled to be set to nil' do
+          project_auto_devops.reload
+
+          expect(project_auto_devops.enabled).to be_nil
+        end
+      end
     end
   end
 end