Cleanup welcome helper method

ee/app/helpers/ee/welcome_helper.rb

@@ -13,7 +13,7 @@ module EE
     end
 
     def in_invitation_flow?
-      redirect_path&.starts_with?('/-/invites/')
+      redirect_path.present? && redirect_path.starts_with?('/-/invites/')
     end
 
     def in_oauth_flow?

ee/spec/helpers/ee/welcome_helper_spec.rb

@@ -42,7 +42,7 @@ RSpec.describe EE::WelcomeHelper do
       '/-/invites/xxx' | true
       '/invites/xxx'   | false
       '/foo'           | false
-      nil              | nil
+      nil              | false
     end
 
     with_them do