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.
