Change boolean value to string in helper

* It appears that passing values to Vue through HTML/HAML requires all
values to be strings. Boolean values get converted to empty strings.

app/helpers/application_settings_helper.rb

@@ -352,7 +352,7 @@ module ApplicationSettingsHelper
         status_delete_self_monitoring_project_admin_application_settings_path,
 
       'self_monitoring_project_exists' =>
-        Gitlab::CurrentSettings.instance_administration_project.present?,
+        Gitlab::CurrentSettings.instance_administration_project.present?.to_s,
 
       'self_monitoring_project_full_path' =>
         Gitlab::CurrentSettings.instance_administration_project&.full_path

spec/helpers/application_settings_helper_spec.rb

@@ -92,7 +92,7 @@ describe ApplicationSettingsHelper do
 
       it 'returns self_monitoring_project_exists false' do
         expect(helper.self_monitoring_project_data).to include(
-          'self_monitoring_project_exists' => false
+          'self_monitoring_project_exists' => "false"
         )
       end
 
@@ -112,7 +112,7 @@ describe ApplicationSettingsHelper do
 
       it 'returns self_monitoring_project_exists true' do
         expect(helper.self_monitoring_project_data).to include(
-          'self_monitoring_project_exists' => true
+          'self_monitoring_project_exists' => "true"
         )
       end