Notification icons: Render empty string for "custom" setting

app/helpers/notifications_helper.rb

@@ -29,7 +29,10 @@ module NotificationsHelper
   end
 
   def notification_icon(level)
-    sprite_icon("#{notification_icon_class(level)}")
+    icon = notification_icon_class(level)
+    return '' unless icon
+
+    sprite_icon(icon)
   end
 
   def notification_title(level)

changelogs/unreleased/254224-notification-icons-missing-icon-for-custom.yml

+---
+title: 'Notifications icon: Render empty string for custom setting'
+merge_request: 42848
+author:
+type: other

spec/helpers/notifications_helper_spec.rb

@@ -10,6 +10,7 @@ RSpec.describe NotificationsHelper do
     it { expect(notification_icon(:mention)).to match('data-testid="at-icon"') }
     it { expect(notification_icon(:global)).to match('data-testid="earth-icon') }
     it { expect(notification_icon(:watch)).to match('data-testid="eye-icon"') }
+    it { expect(notification_icon(:custom)).to equal('') }
   end
 
   describe 'notification_title' do