Add link_url validation for cards when are not header cards

Currently it is not necessary to include the link_url field.

When we display these cards without link_url, they create an empty link that
redirects to the same page. I understand that this is not a desired behavior, so I
think it is better to add a validation in this case and force administrators to add a
link_url when creating a card.

spec/system/admin/widgets/cards_spec.rb

@@ -172,6 +172,7 @@ describe "Cards", :admin do
           href: admin_site_customization_page_widget_cards_path(custom_page))
 
         fill_in "Title", with: "Card for a custom page"
+        fill_in "Link URL", with: "/any_path"
         click_button "Create card"
 
         expect(page).to have_current_path admin_site_customization_page_widget_cards_path(custom_page)