Avoid rendenring an empty list in top links

We've had an accessibility error reported by the Spanish "Portal
Administración electrónica" (PAe). While I can't find any accessibility
rule saying empty lists should be avoided, it looks like some screen
readers report finding lists with 0 items, which is annoying.

spec/components/layout/top_links_component_spec.rb

@@ -0,0 +1,19 @@
+require "rails_helper"
+
+describe Layout::TopLinksComponent, type: :component do
+  describe "#render?" do
+    it "renders when a content block is defined" do
+      create(:site_customization_content_block, name: "top_links")
+
+      render_inline Layout::TopLinksComponent.new
+
+      expect(page).to have_css "ul"
+    end
+
+    it "does not render when no content block is defined" do
+      render_inline Layout::TopLinksComponent.new
+
+      expect(page).not_to have_css "ul"
+    end
+  end
+end