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.

app/models/widget/card.rb

@@ -9,6 +9,7 @@ class Widget::Card < ApplicationRecord
   include Globalizable
 
   validates_translation :title, presence: true
+  validates :link_url, presence: true, if: -> { !header? }
 
   def self.header
     where(header: true)

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

spec/models/widget/card_spec.rb

@@ -17,6 +17,14 @@ describe Widget::Card do
     it "is not valid without a title" do
       expect(build(:widget_card, title: "")).not_to be_valid
     end
+
+    context "regular cards" do
+      it "is not valid without a link_url" do
+        card = build(:widget_card, header: false, link_url: nil)
+
+        expect(card).not_to be_valid
+      end
+    end
   end
 
   describe "#header" do

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)

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