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/sdg_management/homepage_spec.rb

@@ -22,6 +22,7 @@ describe "SDG homepage configuration" do
       click_link "Create planning card"
 
       within(".translatable-fields") { fill_in "Title", with: "My planning card" }
+      fill_in "Link URL", with: "/any_path"
       click_button "Create card"
 
       within(".planning-cards") do
@@ -54,6 +55,7 @@ describe "SDG homepage configuration" do
       click_link "Create header"
 
       within(".translatable-fields") { fill_in "Title", with: "My header" }
+      fill_in "Link URL", with: "/any_path"
       click_button "Create card"
 
       within(".sdg-header") do