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.

db/dev_seeds/sdg.rb

@@ -33,6 +33,7 @@ end
 
 section "Creating SDG homepage cards" do
   SDG::Phase.all.each do |phase|
-    Widget::Card.create!(cardable: phase, title: "#{phase.title} card")
+    Widget::Card.create!(cardable: phase, title: "#{phase.title} card",
+                        link_text: "Link Text", link_url: "/any_path")
   end
 end