Fix help text in wizard with validation errors

There was an edge case where we could access the headings index without
sending the mode parameter in the URL. That meant when sending the
headings form we could send a form with the mode hidden field set to an
empty string. When that happened, the returned text was
`t("admin.budgets.help.#{i18n_namespace}.`, which returned a hash.

Using `multiple` when an empty strin is received solves the issue.

app/components/admin/budgets/help_component.rb

@@ -6,7 +6,7 @@ class Admin::Budgets::HelpComponent < ApplicationComponent
   end
 
   def budget_mode
-    (helpers.budget_mode if helpers.respond_to?(:budget_mode)) || "multiple"
+    (helpers.budget_mode if helpers.respond_to?(:budget_mode)).presence || "multiple"
   end
 
   private

spec/system/admin/budgets_wizard/headings_spec.rb

@@ -81,6 +81,11 @@ describe "Budgets wizard, headings step", :admin do
       expect(page).to have_button "Cancel"
       expect(page).not_to have_button "Add new heading"
       expect(page).not_to have_content "Continue to phases"
+
+      within ".budgets-help" do
+        expect(page).to have_content "Headings are meant"
+        expect(page).not_to have_content "{"
+      end
     end
   end