FIX: creating an automation without script should error (#28752)

This commit ensure we are properly showing an error to the end user and not just a vague 500.

plugins/automation/spec/system/new_automation_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+describe "DiscourseAutomation | New automation", type: :system, js: true do
+  fab!(:admin)
+
+  before do
+    SiteSetting.discourse_automation_enabled = true
+    sign_in(admin)
+  end
+
+  let(:new_automation_page) { PageObjects::Pages::NewAutomation.new }
+
+  context "when the script is not selected" do
+    it "shows an error" do
+      new_automation_page.visit.fill_name("aaaaa").create
+
+      expect(new_automation_page).to have_error(I18n.t("errors.messages.blank"))
+    end
+  end
+end